Bacterial conjugation: Difference between revisions

'''Bacterial conjugation''' is the transfer of genetic material between [[Bacteria|bacterial cells]] by direct cell-to-cell contact or by a bridge-like connection between two cells.<ref name=Baron>{{cite book |last1=Holmes |first1=Randall K. |last2=Jobling |first2=Michael G. | title = Genetics: Conjugation. ''in:'' Baron's Medical Microbiology| chapter = Genetics| editor = Baron S |edition=4th | publisher = Univ of Texas Medical Branch | year = 1996 | pmid = 21413277| url = https://www.ncbi.nlm.nih.gov/books/NBK7908/ | isbn = 0-9631172-1-1 }}</ref> This takes place through a [[pilus]].<ref name=Shreyas>{{cite book|author = Dr.T.S.Ramarao M.sc, Ph.D.  |title = B.sc Botany-Volume-1|year=1991}}</ref>{{Full citation needed|date=June 2024}} It is a [[parasexual cycle|parasexual]] mode of reproduction in bacteria.

It is a mechanism of [[horizontal gene transfer]] as are [[Transformation (genetics)|transformation]] and [[Transduction (genetics)|transduction]] although these two other mechanisms do not involve cell-to-cell contact.<ref name="Griffiths_1999">{{cite book |last=Griffiths |first=AJF |title=An Introduction to genetic analysis |edition=7th |publisher=W.H. Freeman |location=San Francisco |year=1999 |isbn=978-0-7167-3520-5 |url=https://www.ncbi.nlm.nih.gov/books/NBK21942/ |access-date=2023-08-11 |archive-date=2020-02-08 |archive-url=https://web.archive.org/web/20200208101618/https://www.ncbi.nlm.nih.gov/books/NBK21942/ |url-status=dead }}</ref>

Classical ''E. coli'' bacterial conjugation is often regarded as the bacterial equivalent of [[sexual reproduction]] or [[mating]], since it involves the exchange of genetic material. However, it is not sexual reproduction, since no exchange of gamete occurs, and indeed no [[biogenesis|generation of a new organism]]: instead, an existing organism is transformed. During classical ''E. coli'' conjugation, the ''donor'' cell provides a conjugative or mobilizable genetic element that is most often a [[plasmid]] or [[transposon]].<ref name=Sherris>{{cite book |editor1-last=Ryan |editor1-first=K.J. |editor2-last=Ray |editor2-first=C.G. |title=Sherris Medical Microbiology |edition=4th |pages=60–64 | publisher=McGraw Hill |location=New York |year=2004 |isbn=978-0-8385-8529-0 }}</ref> Most conjugative plasmids have systems ensuring that the ''recipient'' cell does not already contain a similar element.

Conjugation in ''[[Escherichia coli]]'' by spontaneous zygogenesis<ref name="pmid12949181">{{cite journal |last1=Gratia |first1=Jean-Pierre |last2=Thiry |first2=Marc |title=Spontaneous zygogenesis in Escherichia coli, a form of true sexuality in prokaryotes |journal=Microbiology |date=1 September 2003 |volume=149 |issue=9 |pages=2571–2584 |doi=10.1099/mic.0.26348-0 |pmid=12949181 |doi-access=free}}</ref> and in ''[[Mycobacterium smegmatis]]'' by distributive conjugal transfer<ref name=GrayTAKrywy>{{cite journal |last1=Gray |first1=Todd A. |last2=Krywy |first2=Janet A. |last3=Harold |first3=Jessica |last4=Palumbo |first4=Michael J. |last5=Derbyshire |first5=Keith M. |title=Distributive Conjugal Transfer in Mycobacteria Generates Progeny with Meiotic-Like Genome-Wide Mosaicism, Allowing Mapping of a Mating Identity Locus |journal=PLOS Biology |date=9 July 2013 |volume=11 |issue=7 |pages=e1001602 |doi=10.1371/journal.pbio.1001602 |pmid=23874149 |pmc=3706393 |doi-access=free }}</ref><ref name=DerbyshireKMGray>{{cite journal |last1=Derbyshire |first1=Keith M. |last2=Gray |first2=Todd A. |title=Distributive Conjugal Transfer: New Insights into Horizontal Gene Transfer and Genetic Exchange in Mycobacteria |journal=Microbiology Spectrum |date=17 January 2014 |volume=2 |issue=1 |doi=10.1128/microbiolspec.MGM2-0022-2013 |pmid=25505644 |pmc=4259119}}</ref> differ from the better studied classical ''E. coli'' conjugation in that these cases involve substantial blending of the parental [[genome]]s.

The process was discovered by [[Joshua Lederberg]] and [[Edward Tatum]]<ref>{{cite journal |last1=Lederberg |first1=Joshua |last2=Tatum |first2=E. L. |title=Gene Recombination in Escherichia Coli |journal=Nature |date=October 1946 |volume=158 |issue=4016 |pages=558 |doi=10.1038/158558a0 |bibcode=1946Natur.158..558L | s2cid=1826960 |doi-access=free |pmid=21001945 }}</ref> in 1946.

[[File:Conjugation.svg|right|thumb|350px|Schematic drawing of bacterial conjugation.]]

# Donor cell produces [[pilus]].

# Both cells synthesize a complementary strand to produce a double stranded circular plasmid and also reproduce pili; both cells are now viable donor for the F-factor.<ref name=Baron />

The [[Fertility factor (bacteria)|F-factor]] is an [[episome]] (a plasmid that can integrate itself into the bacterial [[chromosome]] by [[Homologous recombination#In bacteria|homologous recombination]]) with a length of about 100 [[kilo-base pair|kb]]. It carries its own [[origin of replication]], the ''oriV'', and an origin of transfer, or ''oriT''.<ref name=Sherris /> There can only be one copy of the F-plasmid in a given bacterium, either free or integrated, and bacteria that possess a copy are called ''F-positive'' or ''F-plus'' (denoted F⁺). Cells that lack F plasmids are called ''F-negative'' or ''F-minus'' (F⁻) and as such can function as recipient cells.{{cn|date=February 2023}}

Among other genetic information, the F-plasmid carries a ''tra'' and ''trb'' [[locus (genetics)|locus]], which together are about 33 kb long and consist of about 40 [[gene]]s. The ''tra'' locus includes the ''pilin'' gene and regulatory genes, which together form [[pilus|pili]] on the cell surface. The locus also includes the genes for the [[protein]]s that attach themselves to the surface of F⁻ bacteria and initiate conjugation. Though there is some debate on the exact mechanism of conjugation it seems that the pili are the structures through which DNA exchange occurs. The F-pili are extremely resistant to mechanical and thermochemical stress, which guarantees successful conjugation in a variety of environments.<ref>{{cite journal |last1=Patkowski |first1=Jonasz B. |last2=Dahlberg |first2=Tobias |last3=Amin |first3=Himani |last4=Gahlot |first4=Dharmender K. |last5=Vijayrajratnam |first5=Sukhithasri |last6=Vogel |first6=Joseph P. |last7=Francis |first7=Matthew S. |last8=Baker |first8=Joseph L. |last9=Andersson |first9=Magnus |last10=Costa |first10=Tiago R. D. |title=The F-pilus biomechanical adaptability accelerates conjugative dissemination of antimicrobial resistance and biofilm formation |journal=Nature Communications |date=5 April 2023 |volume=14 |issue=1 |pages=1879 |doi=10.1038/s41467-023-37600-y |pmid=37019921 |pmc=10076315}}</ref> Several proteins coded for in the ''tra'' or ''trb'' locus seem to open a channel between the bacteria and it is thought that the traD enzyme, located at the base of the pilus, initiates membrane fusion.

When conjugation is initiated by a signal, the '''[[relaxase]]''' [[enzyme]] creates a [[nick (DNA)|nick]] in one of the strands of the conjugative plasmid at the ''oriT''. Relaxase may work alone, or in a complex of over a dozen proteins known collectively as a '''[[relaxosome]]'''. In the F-plasmid system, the relaxase enzyme is called TraI and the relaxosome consists of TraI, TraY, TraM and the integrated host factor IHF. The nicked strand, or ''T-strand'', is then unwound from the unbroken strand and transferred to the recipient cell in a 5'-terminus to 3'-terminus direction. The remaining strand is replicated either independent of conjugative action (vegetative replication beginning at the ''oriV'') or in concert with conjugation (conjugative replication similar to the [[rolling circle]] replication of [[lambda phage]]). Conjugative replication may require a second nick before successful transfer can occur. A recent report claims to have inhibited conjugation with chemicals that mimic an intermediate step of this second nicking event.<ref>{{cite journal |last1=Lujan |first1=Scott A. |last2=Guogas |first2=Laura M. |last3=Ragonese |first3=Heather |last4=Matson |first4=Steven W. |last5=Redinbo |first5=Matthew R. |title=Disrupting antibiotic resistance propagation by inhibiting the conjugative DNA relaxase |journal=Proceedings of the National Academy of Sciences |date=24 July 2007 |volume=104 |issue=30 |pages=12282–12287 |doi=10.1073/pnas.0702760104 |pmc=1916486 |jstor=25436291 |pmid=17630285 |bibcode=2007PNAS..10412282L |doi-access=free}}</ref>

A cell culture that contains in its population cells with non-integrated F-plasmids usually also contains a few cells that have accidentally integrated their plasmids. It is these cells that are responsible for the low-frequency chromosomal gene transfers that occur in such cultures. Some strains of bacteria with an integrated F-plasmid can be isolated and grown in pure culture. Because such strains transfer chromosomal genes very efficiently they are called '''[[Hfr cell|Hfr]]''' ('''h'''igh '''f'''requency of '''r'''ecombination). The ''E. coli'' [[genome]] was originally mapped by interrupted mating experiments in which various Hfr cells in the process of conjugation were sheared from recipients after less than 100 minutes (initially using a Waring blender). The genes that were transferred were then investigated.

Since integration of the F-plasmid into the ''E. coli'' chromosome is a rare spontaneous occurrence, and since the numerous genes promoting DNA transfer are in the plasmid genome rather than in the bacterial genome, it has been argued that conjugative bacterial gene transfer, as it occurs in the ''E. coli'' Hfr system, is not an evolutionary adaptation of the bacterial host, nor is it likely ancestral to eukaryotic sex.<ref>{{cite journal |last1=Michod |first1=Richard E. |last2=Bernstein |first2=Harris |last3=Nedelcu |first3=Aurora M. |title=Adaptive value of sex in microbial pathogens |journal=Infection, Genetics and Evolution |date=May 2008 |volume=8 |issue=3 |pages=267–285 |doi=10.1016/j.meegid.2008.01.002 |url=http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf |pmid=18295550 |bibcode=2008InfGE...8..267M |archive-date=2016-12-30 |access-date=2013-04-22 |archive-url=https://web.archive.org/web/20161230121043/http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf |url-status=dead }}</ref>

In addition to classical bacterial conjugation described above for ''E. coli'', a form of conjugation referred to as spontaneous zygogenesis (Z-mating for short) is observed in certain strains of ''E. coli''.<ref name="pmid12949181" />  In Z-mating there is complete genetic mixing, and unstable [[ploidy|diploids]] are formed that throw off phenotypically haploid cells, of which some show a parental [[phenotype]] and some are true [[genetic recombination|recombinants]].

Hyperthermophilic [[archaea]] encode pili structurally similar to the bacterial conjugative pili.<ref name=":2">{{Cite journal |last1=Beltran |first1=Leticia C. |last2=Cvirkaite-Krupovic |first2=Virginija |last3=Miller |first3=Jessalyn |last4=Wang |first4=Fengbin |last5=Kreutzberger |first5=Mark A. B. |last6=Patkowski |first6=Jonasz B. |last7=Costa |first7=Tiago R. D. |last8=Schouten |first8=Stefan |last9=Levental |first9=Ilya |last10=Conticello |first10=Vincent P. |last11=Egelman |first11=Edward H. |last12=Krupovic |first12=Mart |date=2023-02-07 |title=Archaeal DNA-import apparatus is homologous to bacterial conjugation machinery |journal=Nature Communications |volume=14 |issue=1 |pages=666 |doi=10.1038/s41467-023-36349-8 |issn=2041-1723 |pmc=9905601 |pmid=36750723|bibcode=2023NatCo..14..666B }}</ref> However, unlike in bacteria, where conjugation apparatus typically mediates the transfer of mobile genetic elements, such as plasmids or transposons, the conjugative machinery of hyperthermophilic archaea, called Ced (Crenarchaeal system for exchange of DNA)<ref>{{Cite journal |last1=van Wolferen |first1=Marleen |last2=Wagner |first2=Alexander |last3=van der Does |first3=Chris |last4=Albers |first4=Sonja-Verena |author-link4=Sonja-Verena Albers |date=2016-03-01 |title=The archaeal Ced system imports DNA |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=113 |issue=9 |pages=2496–2501 |doi=10.1073/pnas.1513740113 |issn=1091-6490 |pmc=4780597 |pmid=26884154|bibcode=2016PNAS..113.2496V |doi-access=free }}</ref> and Ted (Thermoproteales system for exchange of DNA),<ref name=":2" /> appears to be responsible for the transfer of cellular DNA between members of the same species. It has been suggested that in these archaea the conjugation machinery has been fully domesticated for promoting DNA repair through homologous recombination rather than spread of mobile genetic elements.<ref name=":2" /> In addition to the VirB2-like conjugative pilus, the Ced and Ted systems include components for the VirB6-like transmembrane mating pore and the VirB4-like ATPase.<ref name=":2" />

Bacteria related to the [[diazotroph|nitrogen fixing]] ''[[Rhizobia]]'' are an interesting case of inter-[[Kingdom (biology)|kingdom]] conjugation.<ref>{{cite journal |last1=Pan |first1=Shen Q. |last2=Jin |first2=Shouguang |last3=Boulton |first3=Margaret I. |last4=Hawes |first4=Martha |last5=Gordon |first5=Milton P. |last6=Nester |first6=Eugene W. |title=An Agrobacterium virulence factor encoded by a Ti plasmid gene or a chromosomal gene is required for T-DNA transfer into plants |journal=Molecular Microbiology |date=July 1995 |volume=17 |issue=2 |pages=259–269 |pmid=7494475 |doi=10.1111/j.1365-2958.1995.mmi_17020259.x |s2cid=38483513}}</ref> For example, the tumor-inducing (Ti) plasmid of ''[[Agrobacterium]]'' and the root-tumor inducing (Ri) plasmid of ''A. rhizogenes'' contain genes that are capable of transferring to plant cells. The expression of these genes effectively transforms the plant cells into [[opines|opine]]-producing factories. Opines are used by the bacteria as sources of nitrogen and energy. Infected cells form [[Agrobacterium tumefaciens|crown gall]] or [[Agrobacterium rhizogenes|root tumors]]. The Ti and Ri plasmids are thus [[endosymbiont]]s of the bacteria, which are in turn endosymbionts (or parasites) of the infected plant.{{cn|date=February 2023}}

Conjugation is a convenient means for [[genetic engineering|transferring genetic material]] to a variety of targets. In laboratories, successful transfers have been reported from bacteria to yeast,<ref>{{cite journal |last1=Heinemann |first1=Jack A. |last2=Sprague |first2=George F. |title=Bacterial conjugative plasmids mobilize DNA transfer between bacteria and yeast |journal=Nature |date=July 1989 |volume=340 |issue=6230 |pages=205–209 |doi=10.1038/340205a0 |pmid=2666856 |bibcode=1989Natur.340..205H |s2cid=4351266 }}</ref> plants, mammalian cells,<ref>{{cite journal |last1=Kunik |first1=Tayla |last2=Tzfira |first2=Tzvi |last3=Kapulnik |first3=Yoram |last4=Gafni |first4=Yedidya |last5=Dingwall |first5=Colin |last6=Citovsky |first6=Vitaly |title=Genetic transformation of HeLa cells by Agrobacterium |journal=Proceedings of the National Academy of Sciences |date=13 February 2001 |volume=98 |issue=4 |pages=1871–1876 |doi=10.1073/pnas.041327598 |pmid=11172043 |pmc=29349 |bibcode=2001PNAS...98.1871K |doi-access=free }}</ref><ref>{{cite journal |last=Waters |first=Virginia L. |title=Conjugation between bacterial and mammalian cells |journal=Nature Genetics |volume=29 |issue=4 |pages=375–6 |date=December 2001 |pmid=11726922 |doi=10.1038/ng779 |s2cid=27160 }}</ref> [[diatom]]s<ref>{{Cite journal |last1=Karas |first1=Bogumil J. |last2=Diner |first2=Rachel E. |last3=Lefebvre |first3=Stephane C. |last4=McQuaid |first4=Jeff |last5=Phillips |first5=Alex P.R. |last6=Noddings |first6=Chari M.|last7=Brunson |first7=John K. |last8=Valas |first8=Ruben E. |last9=Deerinck |first9=Thomas J. |date=2015-04-21 |title=Designer diatom episomes delivered by bacterial conjugation |journal=Nature Communications |language=en |volume=6 |article-number=6925 |doi=10.1038/ncomms7925 |issn=2041-1723 |pmc=4411287 |pmid=25897682 |bibcode=2015NatCo...6.6925K}}</ref> and isolated mammalian [[mitochondria]].<ref>{{cite journal |last1=Yoon |first1=Young Geol |last2=Koob |first2=Michael D. |title=Transformation of isolated mammalian mitochondria by bacterial conjugation |journal=Nucleic Acids Research |volume=33 |issue=16 |pages=e139 |year=2005 |pmid=16157861 |pmc=1201378 |doi=10.1093/nar/gni140 }}</ref> Conjugation has advantages over other forms of genetic transfer including minimal disruption of the target's [[Cellular membrane|cellular envelope]] and the ability to transfer relatively large amounts of genetic material (see the above discussion of ''E. coli'' chromosome transfer). In plant engineering, ''Agrobacterium''-like conjugation complements other standard vehicles such as [[tobacco mosaic virus]] (TMV). While TMV is capable of infecting many plant families these are primarily [[Herbaceous plant|herbaceous]] [[dicot]]s. ''Agrobacterium''-like conjugation is also primarily used for dicots, but [[monocot]] recipients are not uncommon.{{cn|date=February 2023}}