What is the significance of conjugation

In the Ti plasmid system, induction of tra genes is therefore dependent on signal molecule mediated repressor inactivation and activator production, which, once initiated, is enhanced by a positive feedback loop provided by AHL synthesis , presumably resulting in a burst of tra gene expression in individual cells harboring the Ti plasmid.

The system can be turned OFF by anti-activators TraM and TrlR under the control of TraR—negative feedback loop and may be modulated by lactonases that can specifically hydrolyze the AHL molecule in response to plant signals Haudecoeur and Faure, Besides the Ti plasmid and two chromosomes, Agrobacterium tumefaciens C58 also harbors another large conjugative plasmid, pAT.

Notwithstanding the lack of obvious signaling molecules involved in F-conjugation module mediated DNA transfer, sensing environmental conditions in combination with the physiological status of the potential donor cell affects the behavior of the cell through a network of regulatory elements for a detailed description see Frost and Koraimann, CPs with F-like conjugation modules are mainly found in the Enterobacteriaceae including pathogenic Escherichia , Salmonella , and Klebsiella species.

Typically, the plasmid encoded transcriptional activator of tra genes, TraJ, is under the negative control of two fertility inhibition elements, FinO and FinP. In populations of donor cells—under optimal conditions promoting growth and cell division—only in few cells 1—10 out of potential donor cells TraJ escapes this negative FinOP mediated control and promotes transcription of tra genes together with the host encoded transcriptional activator ArcA-P Strohmaier et al.

Similarly to other conjugation systems described in this review, a negative feedback-loop exists that mediates shut-off of tra gene expression via the DNA binding protein TraY which has an activating role at low concentrations but can inhibit tra gene expression at higher concentrations.

Other factors that contribute to the shut-off of tra gene transcription or modulate and fine tune this system are extracellular and cellular stress response elements, including the CpxAR two component system, proteases, and the chaperone protein GroEL Zahrl et al.

From studies of many different conjugation systems it has become evident that, even under controlled laboratory conditions, the transition to transfer competence does not occur in all cells of a population Figure 2B.

One example in which this phenomenon has been illustrated nicely is the demonstration of discontinuous patches of gene transfer between donor and recipient cells at the edge of bacterial colonies on semi-solid agar surfaces Reisner et al. Higher magnifications of these zones revealed that these patches correspond to infrequently occurring gene transfer events from some cells of the donor cell population to recipient cells but not from all. This phenomenon was observed in the case of a derivative of the naturally repressed F-like plasmid R1 with an intact FinOP repressor system see above.

These findings are consistent with the observation in liquid media where tra gene expression is low in the presence of plasmid R1 compared to a de-repressed mutant. Since the first observation published by Ghigo that the presence of CPs in bacterial populations induces the formation of biofilms it has become increasingly evident that these microbial communities are hot-spots for social interactions and horizontal gene transfer HGT.

In short, CPs promote biofilm formation and, vice versa, biofilms promote conjugation Molin and Tolker-Nielsen, ; Madsen et al. The underlying gene regulatory mechanisms, however, are largely unknown because tra gene expression studies in biofilms are difficult to perform due to the dynamic nature of biofilms and the associated inherent heterogeneity of cells.

Which donor cells in a biofilm community actually progress via activation of tra gene expression to transfer competent cells is unknown. What can be observed at the single cell level by sophisticated genetic constructs and fluorescence microscopy, however, is the transfer of plasmid DNA into recipient cells and the spread of the CP in the recipient population. In one case, the transfer of the pWW0 TOL plasmid from Pseudomonas putida donor to recipient microcolonies on semi-solid agar surfaces was investigated.

Intriguingly, time-lapse microscopic images revealed that spreading of the CP in the recipient originated from one transfer event between cells contacting each other at the edges of donor and recipient microcolonies. This single transfer event was followed by limited, cell division dependent, spreading of the CP in the recipient colony. Again, similarly to plasmid R1 see above , not all donor cells that were in contact with recipient cells initially transferred the CP, indicating regulatory mechanisms that maintain the OFF state in most of the cells of the donor microcolony Seoane et al.

Regulatory mechanisms including negative autoregulation by a transcriptional repressor of tra genes that could account for a shut-off after an initial burst have indeed been demonstrated for the pWW0 plasmid Lambertsen et al. In analogy to the microcolony situation, limited invasion of recipient cells in E. In theory, CPs should, once established in a bacterial host, represent a burden and generate a fitness disadvantage, resulting eventually in the elimination of the plasmid from a population.

This, however, as evidenced by the persistence of these elements, seems not to be the case. Are there advantages conferred to the host by the CP in the absence of selection for genes that are carried by the CP? There are several studies in which the apparent paradox of the persistence of CPs in the bacterial world has been investigated Modi and Adams, ; Dahlberg and Chao, ; Dionisio et al. One interesting result of such studies was that, CPs such as R1 and RP4 were not lost from bacterial populations even after more than generations of growth without selective pressure.

This was attributed to the fact that such plasmids have a controlled replication system and low copy numbers as well as active partitioning and plasmid stability systems that prevent plasmid loss. An initial minimal fitness cost that was imposed on the E.

Coevolution induced changes were observed in both the evolved host cells and plasmids. Interestingly, evolved plasmid R1 had slightly lower transfer rates in the evolved host than in the ancestral host Dahlberg and Chao, These findings were corroborated by a similar study where it was found that an evolved plasmid R1 even conferred a relative fitness advantage to the original E.

In addition, the original R1 plasmid had no fitness cost in the evolved E. These results and other studies have led to the proposal that CP mediated bacterial conjugation is a coevolutionary process Harrison and Brockhurst, Although not measured directly, the data of Dahlberg and Chao suggest that a major cost of CP carriage is the expression of tra genes.

In line with this proposition is the fact that the activation of tra genes in case of plasmid R1 causes the up-regulation of extracytoplasmic and cytoplasmic stress regulons Zahrl et al. In addition, F plasmid tra gene expression and T4S system assembly causes increased sensitivity to bile salts Bidlack and Silverman, In any case, due to the regulatory regime that keeps tra genes OFF in the majority of donors, metabolic burden and exposure to pilus specific bacteriophages is not evenly distributed within a population but instead restricted to a small fraction of the population.

In this way, possible detrimental and cell threatening effects associated with tra gene expression are limited to a few cells within a population whereas all cells retain beneficial genes and the potential for HGT.

An intrinsically beneficial feature contributing to the persistence of CPs within bacterial populations may be their well documented ability to promote formation of biofilms see above. Interestingly, besides the specific regulatory mechanisms discussed in this review that operate to control tra gene expression, there is a general silencing mechanism in enterobacteria that mediates silencing of laterally acquired genes by H-NS and related proteins Navarre et al.

Although the molecular details of how regulatory networks control tra gene expression are different in the conjugation systems presented in this review, there is a common theme: As a default, tra genes are OFF and whenever positive stimuli are present, not the whole population transits to the ON stage but only a fraction of the cells carrying a conjugative element.

In this way the metabolic burden fitness cost imposed by expression of tra genes and assembly of a cell envelope localized DNA secretion machine a T4SS is carried not by the whole population but distributed to only a few cells within a population. Further studies at the single cell level are needed to reveal whether the transformation of only a fraction of a donor cell population into transfer competent cells is due to a stochastic process or depends on different physiological states such as metabolic conditions, cellular fitness and cell age.

Moreover, positioning of individual cells in structured communities microcolonies or biofilms may influence transition to transfer competence. Undoubtedly intelligent strategies exist to minimize or even eliminate fitness costs associated with the carriage of conjugative elements.

Populations harboring CPs and presumably ICEs can grow and divide largely unaffected by the presence of these elements. At the same time, some cells within a population do become transfer competent and thereby secure the spread and persistence of conjugation modules in many different bacterial species, among them pathogens causing disease in humans, animals, and plants.

Thus, genes carried on the conjugative element, which are beneficial for the host cell in particular habitats e. 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. Althorpe, N. Transient transcriptional activation of the IncI1 plasmid anti-restriction gene ardA and SOS inhibition gene psiB early in conjugating recipient bacteria.

Arthur, D. EMBO J. Auchtung, J. Regulation of a Bacillus subtilis mobile genetic element by intercellular signaling and the global DNA damage response. Babic, A. Efficient gene transfer in bacterial cell chains.

Beaber, J. SOS response promotes horizontal dissemination of antibiotic resistance genes. Nature , 72— Bhatty, M. The expanding bacterial type IV secretion lexicon. Bidlack, J. An active type IV secretion system encoded by the F plasmid sensitizes Escherichia coli to bile salts. Bose, B. Regulation of horizontal gene transfer in Bacillus subtilis by activation of a conserved site-specific protease. Cascales, E. Science , — Chatterjee, A. Antagonistic self-sensing and mate-sensing signaling controls antibiotic-resistance transfer.

Clarke, M. F-pili dynamics by live-cell imaging. Clewell, D. Tales of conjugation and sex pheromones: a plasmid and enterococcal odyssey. Elements 1, 38— Dahlberg, C. Amelioration of the cost of conjugative plasmid carriage in Escherichia coli K Genetics , — Pubmed Abstract Pubmed Full Text. This allows a single strand of dsDNA to be transported to the cytoplasm of the recipient cell. During conjugation, two bacterial cells have to be in contact with each other this contact is established by a pilus and athesins.

For the transfer of genetic material to take place, one of the cells must not have the F-factor. Therefore, one has to be the donor and the other a recipient. The whole process allows the recipient cell to assume characteristics of the donor cell. It's also capable of encoding for proteins required for conjugation in order to transfer genetic material to another cell. For instance, because the recipient cell now a donor cell has the F factor, it's capable of coding for sex pilus involved in cell contact and transfer of genetic material.

As compared to donor bacterial cells with F plasmid, Hfr cells high-frequency recombination are strains in which the F factor is integrated into the chromosome of the host. As previously mentioned, the plasmid in many bacteria is separate to the chromosome of the cell.

In these strains, however, the conjugative plasmid is integrated into the chromosomal DNA. As with F plasmid in other cells as described above the genes of the F factor in these cells is involved in mediating the transfer of genetic material from the donor cell with Hfr to the recipient cell.

This results in recombination in the recipient cell as the genetic material integrates into the chromosome. Although conjugation is common between bacterial cells of the same species, this process has also been shown to occur between members of different genera various bacterial species. Moreover, according to recent studies, this has also been shown to occur between bacterial cells prokaryotes and eukaryotic cells. In particular, this behavior has been recognized among members of the class Alpha proteobacteria that have a parasitic or symbiotic relationship with eukaryotic hosts.

According to studies, this occurs through mechanisms that resemble normal bacterial conjugation under the regulation of bacterial T4SS IV secretion system. While the process is yet to be fully understood, it's suspected that T4SS pilus, coded by the bacteria, create pores on the barriers of the host cell by piercing through it. This allows for the direct injection of genetic molecules along with some protein material into the cytoplasm of eukaryotic cells. The internalization of these materials has also been suggested to occur through the involvement of host receptors f endocytosis.

Genetic molecules released on the surface of the cell are not injected into the cells of the host but rather internalized through host receptor mechanisms as well as endocytosis. Here, nuclear important machinery of the host contribute to integration of bacterial DNA. Conjugation is a form of horizontal gene transfer HGT given that it involves the transfer of genetic material between bacteria and even between bacteria and other organisms.

As compared to asexual reproduction through binary fission etc, conjugation allows for genetic material to not only be transferred between members of the same species but also between different genera and even organisms. One of the biggest advantages of this process is that it facilitates evolutionary adaptation of the organism to different environments and habitats.

Through genetic transfer, bacteria are able to gain new characteristics that allow them to adapt and thus survive in environments they may not have survived in before. A good example of this includes bacteria becoming resistant to antibiotics which allow them to continue thriving. See: How do antibiotics kill bacteria? Conjugation has made it possible for scientists to study evolutionary mechanism facilitated by conjugation processes.

In the process, they have also been able to study how bacterial species and other microbes develop antibiotic properties which have in turn made it possible to develop effective treatments. Return to Bacterial Transformation , Transduction. A separate, non-chromosomal DNA ring, known as an F-plasmid, is separated into two strands, and one of them transferred to the recipient bacteria.

Both strands are replicated by DNA polymerases, resulting in both bacteria having a copy of the F-plasmid Griffiths, Conjugation is used in nature to share beneficial genetic material between bacteria, such as antibiotic resistance. However, manually inserting genes into the F-plasmid would allow for scientists to have bacteria transfer almost any gene to other cells, including our AMP kill switch.

By creating cells that can conjugate the kill switch-containing plasmid into non-host microbes, we could then activate that kill switch, disrupting the membranes of, and ultimately killing, those cells. Protegrin-1 also displays antimicrobial activity towards fungi as well, meaning that two separate infection types that would normally require different treatments could be dealt with via a single intercellular technique Kokryakov, Utilizing conjugation to pass this kill switch between cells has many benefits.

All the genes required for conjugation to occur are found within the F-plasmid, meaning that once a non-host cell is conjugated with and receives the F-plasmid, it too can further conjugate the kill switch into other microbes Griffiths, Once spread to multiple non-host microbes, the kill switch promoter could then be induced by outside means, such as the addition of a molecule into the body resulting in mass non-host cell death , or induced statically within the body by a microbial activity to which the promoter is sensitive.

It is important to note that the trigger for kill switch activation could be designed before the kill switch is conjugated, and this could be tailored to the specifics of the situation at hand.

Knowing the type of invading bacteria or fungi, including their preferred ecological niche or intracellular chemical pathways, could allow the promoter of the kill switch to be induced by a process specific to that microbe, removing the need for an outside promoter inducer. This would allow the kill switch to be more specific to the microbe, resulting in a better treatment of that infection.