How can plasmids be transferred

Therefore, we examined the potential of natural E. We used a standard E. In the environment, naked DNA can be naturally supplied from dead cells to neighboring cells within the same habitat or microenvironment. Therefore, it is worth investigating the possibility of HPTT in a closed system under some feasible conditions. Freeze—thaw is a common process in the handling of foodstuffs and also occurs in nature. Freeze—thaw treatment of E. This treatment of condensed suspensions of mixed E.

Biofilms are thought to be suitable environments for in situ transformation because living and dead cells coexist in close proximity, and DNA released from dead cells often accumulates around living cells.

In addition, as described above, because E. Liquid cultures of the same strains in LB broth produced no or few transformants, suggesting the importance of SA biofilm formation for plasmid transfer.

Essentially, the same phenomenon occurred in SA biofilms on food-based media Ando et al. By assessing combinations of several strains and plasmids for horizontal plasmid transfer, the E. Further studies revealed that this phenomenon exhibits some specific characteristics: 1 promotion by proteinaceous factor released from CAG Etchuuya et al.

With respect to 1 , a later study revealed that these proteinaceous factors include a P1 vir phage particle or a derivative thereof and that externally added P1 vir phage can reproduce horizontal plasmid transfer between E.

This phenomenon was also largely DNase-sensitive, suggesting that a large part of this plasmid transfer is due to transformation despite the involvement of P1 phage.

The transformation mechanism of P1 vir phage-induced plasmid transfer may be due to phage infection or spontaneous awakening of lysogenized phage in plasmid-harboring cells, leading to cell lysis and subsequent intracellular plasmid DNA release in a usable form for transformation. Although such a mechanism is generally feasible, there have been few clear demonstrations of it in E. A recent study by Keen et al. With respect to 2 , the bp sequence on pHSG is not homologous to the part of the P1 phage genome sequence.

This sequence is often found in databases among general cloning vector sequences but not in any natural source. With respect to 3 , this high-frequency transfer cannot be explained by the simple PT ability of CAG and other strains used because simple PT in those strains under the equivalent culture condition was 10 5 —10 2 times less frequent Etchuuya et al.

It was, therefore, suggested that a CAGderived proteinaceous factor, with size estimated between 9 and 30 kDa Etchuuya et al. This factor presumably assists in DNA uptake by recipient cells, probably in combination with the bp sequence on the transforming DNA.

Lastly, with respect to 4 , later genome-wide screening studies for recipient genes involved in HPTT suggested that multiple genes participate in the mechanism Kurono et al. These include those that have not been reported to be involved in natural or artificial transformation in E.

Overall, these results point toward an unknown, complex mechanism of phage-induced, high-frequency HPTT that may partly share the pathway of natural transformation. Several combinations of ECOR strains were co-cultured in liquid media, resulting in DNase-sensitive horizontal transfer of natural antibiotic resistance genes Matsumoto et al. Plasmid isolation from these new transformants demonstrated horizontal plasmid transfer between ECOR strains Matsumoto et al. Moreover, we discovered that 6 of 12 combinations of the ECOR strains, some of which produce no plaque-forming phages Shibata et al.

Overall, these data suggest that some phage- and conjugation-free transformation mechanism s also naturally exist in some E. AB and AB can be a pathway for producing multidrug-resistant natural E.

As we described previously Maeda et al. High-frequency HPTT described in this article may involve not only the pore-forming mechanism but also a part of the competence gene functions and possibly another unknown mechanism, as mentioned above.

Because bacteriophages are one of the most abundant organisms in the biosphere and ubiquitous in the environment Clokie et al. Overall, our results and related previous data indicate that multiple mechanisms induce transformation-type HGT in E. Therefore, transformation-type HGT can contribute to the spread of antibiotic resistance genes and emergence of multidrug-resistant bacteria in the real environment outside laboratories. Further studies are required to understand the precise role and contribution of transformation-type HGT in spreading antibiotic resistance.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. We are grateful to Enago www. Anderl, J. Role of antibiotic penetration limitation in Klebsiella pneumoniae biofilm resistance to ampicillin and ciprofloxacin.

Agents Chemother. Ando, T. Horizontal transfer of non-conjugative plasmid in colony biofilm of Escherichia coli on food-based media. World J. Asif, A. Revisiting the mechanisms involved in calcium chloride induced bacterial transformation. Bauer, F. Transformation of Escherichia coli in foodstuffs.

Baur, B. Genetic transformation in freshwater: Escherichia coli is able to develop natural competence. PubMed Abstract Google Scholar. Blesa, A. Contribution of vesicle-protected extracellular DNA to horizontal gene transfer in Thermus spp. Brady, G. New cosmid vectors developed for eukaryotic DNA cloning. Gene 27, — The kinase subdomain includes the phosphorylatable histidine- and glycine-rich segments which may serve as nucleotide binding sites [ , ].

The kinase activity is critical for tumorigenesis since mutation of His results in avirulence []. The C-terminal receiver domain, homologous to N-terminal half of the response regulator VirG, seems to negatively regulate VirA kinase activity.

Removal of this module increased kinase activity []. The coupling of transfer frequency to environmental conditions that this illustrates provides a paradigm which may be found in other systems where cell density or nutritional state determines the level of conjugative activity observed.

Control of Ti vir gene expression. Activation occurs by signalling through phosphorylation of VirA which is then relayed to VirG. VirG—P activates the vir promoters. Regulatory genes, black; DNA processing genes, light grey; mating pair formation, diagonal hatching. The T DNA that is transferred is about 20 kb away to the left.

VirA activates VirG in response to high levels of acetosyringone. To respond to low levels of acetosyringone VirA requires ChvE and monosaccharides released from plant wound sites. This chromosomally encoded virulence protein is required for sugar enhancement of vir gene induction by low levels of acetosyringone. VirA and ChvE interact directly [ , , — ]. It is proposed that VirA has three states of activity: off, standby, and on [96].

ChvE bound to monosaccharide should interact with the periplasmic domain of VirA, thereby altering the repressing or nonfunctional conformation and placing VirA on standby, poised to respond to acetosyringone. In the absence of sugars only a few VirA molecules randomly achieve the standby conformation and they can only respond if acetosyringone is present at a concentration high enough to saturate VirA.

Transmembrane domains TM seem to be important in signalling [ — ]. The proposed standby position for VirA may involve an interaction of TM1 with TM2, causing a conformational shift that exposes the domain responsible for interactions with acetosyringone molecules.

Although separate phenol binding proteins have been identified, recent genetic evidence suggests that VirA protein directly senses acetosyringone [ , ]. Transfer of different Ti plasmids into isogenic chromosomal backgrounds showed that the phenolic sensing determinant is Ti plasmid-specific and subcloning indicated that the virA locus determines which phenolic compounds can function as vir inducers. This suggests that VirA directly senses the phenolic compounds for vir gene activation.

The region distal to TM2, designated a linker domain, seems to be required for VirA to respond to acetosyringone [ , ].

The way in which VirA integrates the signals from these two classes of compound should provide some general lessons in the nature of response elements.

VirG binds to vir boxes in promoter regions of all vir genes, activating their expression when phosphorylated by VirA [ , ].

It consists of at least two domains: the N-terminal signal receiving domain, and C-terminal DNA binding domain [ , ]. The N-terminal domain contains a conserved aspartic acid at position 52, which is phosphorylated by phospho-VirA [].

The C-terminal domain mediates binding to a family of similar sequences designated vir boxes upstream of each vir promoter [ , ].

As for other response regulators it appears that phosphorylation of VirG stimulates DNA binding ability by increasing affinity of receiver domains for each other to form dimers []. Acidity is one of three environmental stimuli that activate the expression of the vir regulon. It may directly stimulate activity of VirA, and it stimulates the expression of virG [ , , ]. VirG is expressed from two promoters: P1 and P2. P1 promoter is activated by phospho-VirG [ 96 , , ] and is also induced by phosphate starvation, probably by a homologue of E.

Induction by acidity occurs at P2 and does not require any Ti-encoded proteins []. Because the induction of virG by acidity and phosphate starvation does not require VirG, these stimuli were viewed as necessary to establish a pool of VirG sufficiently large to induce the vir regulon. This pool of VirG, upon phosphorylation, can strongly express virG in a positively autoregulated fashion.

One candidate may be ChvI protein since chvI null mutants are completely deficient in P2 induction [ , ]. However, so far it has not been possible to demonstrate P2 regulation by ChvI in E.

The induction of the vir system in response to chemicals produced from wounded plants uses a two-component regulatory system consisting of sensor and response regulator as for many other environmental sensing systems such as chemotaxis, nitrogen assimilation and osmoregulation. The signal transduction involves phosphorylation reactions which activate positive regulators of gene expression. As with the Tra system this again illustrates how the Ti plasmid has recruited a ubiquitous regulatory mechanism to control its gene transfer system.

A number of Enterococcal plasmids which are all bigger than 25 kb are transferred very efficiently in liquid cultures — as high as 0. The most extensively studied plasmids from this group are: pAD1 60 kb , encoding hemolysin-bacteriocin production and UV resistance, pCF10 54 kb , encoding tetracycline resistance, and pPD1 59 kb , a bacteriocin producer. Plasmid-free recipients secrete multiple sex pheromones that trigger the donor cell to express transfer functions.

Thus pheromone accumulation indicates to donors that recipients are in close proximity. Pheromones are short hydrophobic peptides excreted in tiny amounts. A given pheromone specifically activates the conjugative transfer system of the corresponding plasmid, e. When a plasmid is acquired, secretion of the related pheromone is prevented, while unrelated pheromones continue to be produced [ — ].

The donor response to pheromone is production of a probable membrane-spanning protein [] which promotes aggregation with recipients via the receptor Enterococcal binding substance EBS , allowing formation of a mating channel which enables transfer. The regions are similar in nucleotide sequence, orf organisation, deduced sequence of amino acids and molecular profile, such as hydrophobicity, for each protein [ , ].

The origin of transfer oriT of pAD1 is located within repA []. Expression of functions relevant to transfer is controlled by negative and positive circuits Fig. Control of pheromone-regulated Gram positive transfer systems as illustrated by pCF The key features of control are the combined action of a number of positive and negative regulators, complex antitermination control and repressors of pheromone production and activity.

Regulators are shown in black; DNA replication functions, light grey; and aggregation functions as diagonal hatching; surface exclusion, dark grey. The latter effect is achieved by peptides that act as competitive inhibitors of the corresponding pheromone []. Thus cells harbouring pAD1 secrete a substance iAD1, encoded by iad , which specifically inhibits the ability to respond to the pheromone cAD1 and the same applies to the other plasmids.

These pheromone competitors may reduce the sensitivity of donors to exogenous pheromone, so that the system will be switched on only when the recipient is very close to a donor cell.

Pheromone internalisation is essential for induction of the pheromone response []. This is achieved by pheromone binding proteins which show significant sequence similarity to bacterial oligopeptide binding proteins OppAs [ , , , ] which must work together with the chromosomal oligopeptide permease system.

In the absence of active pheromone, negative control prevents expression of the majority of the transfer genes. When pheromone enters from outside, negative control is abolished and a distinct positive mechanism upregulates the expression of transfer genes.

It is suggested that the intracellular destination of the imported pheromone is a ribosome translating the polycistronic mRNA whose synthesis initiates at prgQp since affinity chromatography with cCF10 isolated what was deduced to be a complex of ribosomal proteins and prgQ transcript [ , ]. This prgQ region, encoded between prgX and prgA , is needed for expression of prgB , encoding aggregation substance Asc10, but functions only in cis and only if it is in the same orientation as prgB [] despite being 10 kb upstream.

Activation of prgB requires both prgS and prgQ. Transcription of prgQ yields a shorter product, Q s , nt long, in the absence of pheromone and a longer product, Q L , nt long, which is pheromone-inducible in a PrgS-dependent way. It may be this longer transcript or the event that leads to this transcript which is directly involved in the positive regulation. Both prgQ transcripts should be translated to give iCF Recent observations have indicated translational control by nascent polypeptides in other systems [].

In this system it is proposed that the nascent iCF10 modulates ribosome movement on the transcript so that only Q s is made but that cCF10 displaces iCF10 from the ribosome [] , allowing readthrough transcription or increased mRNA stability.

This in turn results in activation of prgB by a localised, possibly intramolecular, interaction which is yet to be elucidated. The product of prgQ responsible for positive control may thus be an RNA []. Analysis of additional elements is complicated by overlap of functions. While prgN and prgX may be also directly involved in negative control referred to in [] , inactivation of prgW abolishes negative control indirectly because of its involvement in transcription of prgZ and prgY.

The same region is required for negative control, signalling prgZ , and replication, so PrgW might be a multifunctional protein and pheromones might be involved in both replication and transfer control. The way that the conjugative plasmids of Enterococci have developed an interaction with their host so that they are able to sense bacteria which do not already carry a plasmid identical to themselves is unique among plasmids studied to date.

This specialisation to a particular group of hosts may be a factor which limits the host range of these plasmids. The regulatory system consists of many levels which are superimposed to achieve this tight control of expression of the transfer genes. In addition there may be a parallel with the Ti plasmids since there are suggestions that many of the components of the pheromone-inducible conjugation system may also play a role in the host-parasite interactions in which Enterococci are involved [].

The possible overlap of transfer and replication functions as the target for pheromone action could provide a means of developing pheromone analogues which interfere with plasmid maintenance and spread in a clinical context. Some Staphylococcal plasmids are 40—60 kb in size and transfer with low frequency 10 4 —10 6 transconjugants per donor on solid surfaces [ , ].

The conjugative transfer region was localised by transposon mutagenesis []. DNA sequence and transcriptional organisation of the transfer region trs [] identified 14 orfs over a kb region which are likely to be functional genes trsA — trsN. A clone containing trs alone cannot be transferred independently and no candidate oriT was found within or contiguous to trs. An additional orf , designated nes nicking enzyme of staphylococcus , located Its predicted amino acid sequence shows similarity to known relaxases.

Nes can generate a single strand nick at oriT. Location of oriT and nes far away from the trs cluster is unique in studied plasmids and may reflect the recent insertion of foreign DNA.

Conversely transcription and conjugation frequency increased in the presence of excess target trsL. This suggests a complex transcription pattern which has yet to fully worked out. TrsN seems to modulate rather than turn off and on gene expression, so the products are made at the appropriate time and level. This suggests the existence of various complex feedback loops and that trsN itself may be regulated. It is not clear if trsN responds to trs gene products or if there are signals external to the cell that trigger or modulate conjugative functions.

The complete nucleotide sequence of the transfer region from pSK41 was recently reported []. Streptococcal broad host range conjugative plasmid pIP Region A contains a functional ori T site similar to oriT s of Gram-negative bacteria [ , ] and six contiguous open reading frames orf1—6 immediately downstream of this oriT site.

Moreover orfs3—6 were able to complement mutations in trans , whereas orf2 could not. Thus despite similarities in oriT and relaxase, the uniqueness of orfs2—6 and their putative products suggest that there may be fundamental differences between transfer systems of Gram-positive and Gram-negative bacteria.

Conjugative transposons combine features of transposons, plasmids and bacteriophages [ , ]. They can excise from and integrate into DNA like transposons, although by a different mechanism from that of well-studied Tn 5 and Tn 10 : they transpose through covalently closed circular intermediates and do not duplicate the target site on integration into DNA. Conjugative transfer also occurs via a covalently closed transfer intermediate, in which respect they resemble plasmids [ — ]. However, their excision and integration resembles excision and integration of temperate bacteriophages and some of the encoded integrases show sequence similarity to members of the lambda integrase family [].

They are found in both Gram-positive and Gram-negative bacteria. The main types are described below. Tn It carries the same Tc r gene, tetM , which encodes a ribosome protection type of resistance protein, as Tn 60 kb , found in Streptococcus pneumoniae [ , , ]. The region essential for transfer of Tn has been mapped and sequenced [ , ]. None of the predicted products of this region has significant sequence similarity to sex pilus proteins of conjugative plasmids and since the number of genes is small, this transfer system may be simpler than that of F or RK2 and may lack a sex pilus.

A small cis -acting fragment that can mobilise a non-transferable plasmid when an intact transposon is present in trans , and should therefore encode oriT , contained four sequences resembling either RP4 or F oriTs []. One orf in the transfer region showed significant similarity to MbeE mobilisation protein of ColE1 []. Thus single-stranded DNA may be transferred during conjugation.

Transfer of Tn is stimulated by tetracycline 10—fold [ — ]. This could be the indirect result of antibiotic-induced stress response. However, because int and xis are located downstream of tetM , stimulation of tetM transcription by tetracycline may result in increased readthrough of int and xis and this could result in increased transposition frequency. It is suggested that there is a low level of constitutive transcription from the promoter upstream of traA and that TraA is a transcriptional activator that upregulates its own transcription and xis -Tn transcription as well as conjugation genes.

In this way transposition and therefore indirectly transfer could be under dual control as observed for the transfer genes of Ti plasmids. In Gram-negative anaerobes, especially Bacteroides , a group of transposons completely unrelated to Tn was found [ , ]. They are much bigger: 65 kb to over kb [ , ].

Like Tn they have a circular transfer intermediate [ , ]. They can excise and mobilise unlinked integrated elements see Section 6. Most of them carry a ribosome protection type of Tc r gene tetQ and they possess a complicated regulatory system that senses tetracycline and controls transfer functions.

The region of the Bacteroides transposon Tc r Em r DOT which is necessary and sufficient for conjugal transfer has been localised to an kb segment []. The oriT region is in the middle of the transposon [].

The ability to mobilise a plasmid in cis suggests that after a nick has been introduced at oriT , a single strand is transported through the mating pore. The oriT has no sequence similarity to oriTs of conjugal E. In addition, the genes encoding transfer proteins are localised at least 3 kb away from the oriT region. The orf adjacent to oriT encodes a regulatory protein RteC, which controls expression of the tra genes. Short exposure of Bacteroides donor carrying Tc r Em r DOT to low levels of tetracycline stimulates self-transfer by 10 fold [ , ].

This stimulation is probably not due to stress caused by tetracycline inhibition of protein synthesis since the nontoxic analogue chlortetracycline stimulates transfer as well. Tetracycline stimulates fold transcription of the operon containing tetQ and regulatory genes rteA and rteB Fig. RteA has amino acid sequence similarity to the sensor protein of two component regulatory systems [] , but its role in control of transfer is not established [].

Control of conjugative transposon transfer genes. Expression of the transfer genes is switched off by an as yet unidentified repressor. On addition of inducing antibiotic, rteA and rteB are switched on. This in turn switches on rteC.

The combination of these positive elements counteracts the action of the repressor in a way that is not fully understood. RteB has amino acid sequence similarity to the activator protein of two component systems and so may function with RteA.

It is essential for transfer [ , ], and activates rteC , a downstream gene []. It seems also to act as antirepressor to counter the effect of an as yet unidentified repressor, which normally prevents transfer genes from being expressed [] : the transfer region of Tc r Em DOT, cloned in the absence of both rteABC and the putative repressor, can transfer constitutively. RteC is essential for self-transfer but is not necessary for mobilisation of co-resident plasmids. It seems unlikely that this complex tetracycline response can have evolved in the time since the first clinical use of this antibiotic.

Tetracycline produced in nature by actinomycetes is unlikely to contribute to evolution of this system because the two organisms live in different niches. Another possibility is that tetracycline is not the natural inducer, but resembles a plant phenolic compounds which is the real inducer. Because Bacteroides conjugate only on solid surfaces, sensing the presence of plants could signal the presence of a suitable surface for mating [].

One of the attachment sites has been identified within pheV , the structural gene for tRNA phe. The transposon encodes a PTS-dependent sucrose fermentation pathway. It seems to be the first conjugative transposon identified in enteric bacteria.

Previously there was only one report of a putative conjugative transposon, R, found in Proteus rettgeri [] , but it has not been well documented. NBUs, nonreplicating Bacteroides units 10—12 kb , are integrated elements that share high homology in an internal region containing mobilisation genes mob and oriT [ — ].

Excision and circularisation of NBUs are triggered by RteB provided by conjugative transposons [ , ]. Expression of rteB is induced by low levels of tetracycline, so that the presence of a conjugative transposon and exposure to tetracycline are necessary for excision and mobilisation of NBUs. NBUs encode only a single mobilisation protein that is capable of binding to oriT , nicking, and initiating transfer of a single stranded copy through a mating pore provided by the conjugative transposon [].

The integrase gene, intN1 , of NBU1 is expressed constitutively. They can be transferred from Bacteroides to E. Conjugative transposons contribute to the spread of antibiotic resistance genes in clinically important bacteria like Bacteroides and Gram-positive cocci. Transfer of many of Bacteroides conjugative transposons is stimulated by low concentrations of antibiotics. Thus antibiotics not only select for resistant strains but also stimulate transfer of resistance genes.

Thus use of antibiotics at sublethal concentration may have a greater effect on resident microflora than had been previously thought. A number of other systems are notable for a variety of reasons although few molecular details are available about control mechanisms. The transfer region of the IncI1 plasmid R64 kb plasmid from S. The region is not simply contiguous transfer functions since stable inheritance genes have also been found within the segment []. It encodes two pilus types as well as auxiliary functions such as primase.

The thick pili produced are needed under all conditions while the thin pili are only needed in liquid matings where mating pairs are more liable to fall apart due to motion of donor and recipient.

Based on the complete sequence of the pil region it is proposed that the thin pili belong to the type IV family, which are more usually associated with attachment of pathogenic bacteria to eukaryotic cells []. There are multiple versions of the pilV gene encoding the pilin for the thin pilus. An unusual control mechanism allows sequential expression of different pilV genes []. Termed the shufflon, the DNA rearrangements are promoted by a series of seven bp repeats which are acted upon by a site-specific recombinase of the integrase family [].

The fourth segment encodes just one possible C-terminus. The different pilus proteins allow invasion of different host species so that these rearrangements control the transfer into specific hosts. R64 shows positive regulation through two putative polypeptides, TraB and TraC, which are needed for expression of a number of transfer-associated functions including the gene for the thin pilus and the primase gene [].

The oriT region consists of oriT and two genes, nikA and nikB which are transcribed from a promoter in the oriT region and are thus autoregulated by the relaxosome assembled by NikA and NikB []. Like the IncP plasmids the IncN plasmids appear to be always transfer proficient.

The genes required are encoded in a kb region, comprising 14 tra genes: three for relaxosome assembly and 11 for Mpf, the latter being similar to the IncP trb genes, the Ti virB genes and the Bordetella ptl genes []. Transfer proficiency without over-burdening the host is achieved by two repressor genes, korA and korB , which are part of the tra region which is organised as divergent operons from a pair of closely spaced promoters between the korB - kikA operon and the traL - korA - traCDNEOFG operon.

There is thus autogenous regulation of the genes for the conjugal mating pore. No regulation is observed unless both genes are functional, which provides a means of coordinating both operons, but it is not known if both proteins or just one bind to DNA.

KorB appears to consist of a direct duplication of a polypeptide with similarity to the HN-S family of histone-like regulatory proteins []. Regulation of the related system encoded by IncW plasmids has not been studied extensively but it is known that TrwA, one of the three relaxosome proteins, provides an autogenous control circuit by binding near oriT which is also the location for the trwAp [].

A number of small mobilisable plasmids also reinforce the principle of autogenous control of the genes for relaxosome proteins. The promoters for these genes are tightly clustered in the oriT region, two firing towards mobAB and one towards mobC.

Binding of the relaxosome proteins to oriT causes autogenous regulation. Mutants in the mobA N-terminal region or in mobC cause derepression. The MobB protein also influences the proportion of the plasmid which exists in the relaxosome and the length of time that the plasmid remains in this state and therefore is a determinant of transfer frequency [].

The mob region of the Recent data have confirmed the existence of autogenous circuits controlling mob gene expression and demonstrated the importance of host protein IHF in determining mobilisation frequency []. On the other hand the mbe region of ColE1, which is very similar to other colicin-producing plasmids such as ColK and ColA, shows no evidence of autoregulation [].

Where transfer genes are found in small clusters rather than a single block, the clustered genes generally have a common function. A scenario of gene recruitment by functional cassette can be envisaged even if the original function of the gene block was not to promote conjugative transfer. One block of genes that was clearly acquired as an already assembled unit is the mpf functions found in IncP, Ti, IncN and IncW but related to the ptl block of B.

There have been some gene rearrangements which may have regulatory or functional significance, for example the juxtaposition of the virB11 homologue to the start of the operon in the IncP plasmids and Ti plasmid Tra system. Regulation has been tagged onto the front of this region in the IncP plasmids through addition of trbA as well as acquisition of a KorB-regulated promoter.

In the case of the IncN system, one of the two regulatory genes, korA , is embedded between the first two genes of the operon and is not found in related operons.

The recruitment or loss of the regulatory genes may be better understood when a larger number of related systems have been sequenced.

The simplest system of regulation found is that of autogenous regulation, in which genes within the blocks of transfer functions shut down expression of the genes once the transfer apparatus is synthesised.

Plasmids with such systems appear to be constantly ready for transfer and the systems so far studied tend to be quite broad in their host range. Perhaps the constant availability of new hosts provides the plasmid with a reason to be perpetually ready for transfer. The assembly of such autogenous circuits is clearly logical in the case of genes for relaxosome proteins transcribed from promoters in the oriT region when these proteins assemble to form the relaxosome.

It is less clear how an autogenous circuit controlling the mpf genes required for pilus assembly can sense when an appropriate number of functional pili are present. The next level of regulation is that exhibited by F-like plasmids in which the genes are normally switched off. The chance of the genes being switched on is small so either a long period of time is required or a large population to ensure the appearance of transfer-proficient bacteria.

If such a bacterium encounters a recipient then transfer occurs and the recipient remains transfer proficient for some time. This allows exponential spread throughout a new population of plasmid-negative bacteria.

Such a system of regulation may be appropriate for bacteria living in the mammalian gut where only occasionally do the bacteria encounter large new populations of potential recipients. The environment of the gut may be too rich with a multitude of bacteria species to allow signalling by a pathway similar to that found in Agrobacterium. The perpetuation of the transfer proficient state by the process of transfer itself seems like a good way of controlling transfer gene expression in response to the presence of appropriate recipients.

An alternative strategy for achieving the same end is represented by the pheromone-responsive plasmids in which potential recipients produce a pheromone that stimulates production of aggregation substance. The plasmid itself actually shuts down internal production of the pheromone to which it responds so that it only switches on transfer genes when there are appropriate recipients present. The other systems surveyed show a close integration of phenotype and transfer frequency. The Ti plasmids respond both to potential recipients, to donor population density and to nutrient availability.

Transfer of the conjugative transposons is specifically stimulated by the antibiotics to which they confer resistance. Once a regulatory system has evolved to deal with, for example, the presence of a pollutant or an antibiotic, genes that link the activity of these transfer processes to environments where the genes are selected will tend to have an advantage because they will over replicate and spread when the environment is right but will minimise their effect on their host when the inducing conditions are absent.

The detailed knowledge that is now accumulating allows us to appreciate what factors are likely to stimulate or limit plasmid spread.

Certain types of antibiotic therapy may promote spread of resistance into other members of a mixed community and leave a potential problem for the future even if the short-term effect is to reduce infection.

Developing screens for compounds which result in derepression of transfer genes could lead to ways of making plasmid carriage a disadvantage. In some extreme cases, for example the IncP plasmids, excess transfer gene expression has been shown to be lethal, so such agents could have a dramatic effect on survival of plasmid-positive bacteria.

The current problems with plasmid borne traits may lead to consideration of antibacterial strategies which would have been discounted years ago. We hope that the material covered in this review may help to trigger such novel concepts. We acknowledge the help of many colleagues for supply of information and useful suggestions.

Frost L. Ippen-Ihler K. Skurry R. Google Scholar. Firth N. Lanka E. Wilkins B. Howard M. Nelson W. Matson S. Sherman J. Kupelwieser, G. Zechner E. Renner W. Fratte R. Jauk B. Koraimann G. Kingsman A. Willetts N. Di Laurenzio L. Paranchych W. Ziegelin G. Pansegrau W. Lurz R. Dempsey W. Mullineaux P. Basic Life Sci. Koraimann C. Koronakis V.

Schlager S. Soderbom F. Wagner E. One example are bacteriophages that attach to bacterial membranes and inject their genetic material into the cell. Once inside, phages can follow one of two different life cycles: lytic or lysogenic. Lytic phages hijack the bacterial hosts machinery to make more viral particles. Eventually the cell lyses releasing the newly formed viral particles that can infect other bacteria.

The integrated phage remains dormant until it is triggered to enter the lytic cycle. During both of these life cycles bacterial DNA can be accidentally packaged into the newly created phages.

Transfer of this DNA to another cell is referred to as transduction. To do this scientists commonly use phagemids , a DNA cloning vector that contains both bacteriophage and plasmid properties. Scientists also use transduction to introduce foreign DNA into eukaryotic cells, like mammalian cell lines. You can find all kinds of different lentiviral and AAV plasmids as well as ready-to-use viral preparations at Addgene.

For more information on viral vectors, including transduction download our Viral Vectors eBook. Conjugation was the first extensively studied method of gene transfer and was discovered in by Joshua Lederberg and Edward Tatum when they observed genetic recombination between two nutritional deficient E.

Plasmids have a wide range of lengths, from roughly one thousand DNA base pairs to hundreds of thousands of base pairs. When a bacterium divides, all of the plasmids contained within the cell are copied such that each daughter cell receives a copy of each plasmid.

Bacteria can also transfer plasmids to one another through a process called conjugation. Scientists have taken advantage of plasmids to use them as tools to clone, transfer, and manipulate genes.