Biological Sciences
Bacterial Conjugation
Bacterial conjugation is a process by which bacteria transfer genetic material, such as plasmids, from one cell to another. This transfer occurs through direct cell-to-cell contact facilitated by a specialized structure called a pilus. Bacterial conjugation plays a significant role in the spread of antibiotic resistance and genetic diversity among bacterial populations.
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12 Key excerpts on "Bacterial Conjugation"
eBook - PDF- Edward Bittar(Author)
- 1998(Publication Date)
- Elsevier Science(Publisher)
Conjugation DNA can also be transferred between cells by conjugation (Clewell, 1993). Conjugation requires functions encoded on certain plasmids. Plasmids are DNA molecules that can replicate independent of the host chromosome. Plasmids are present in most species, but not all strains, of bacteria. Most plasmids are small, from about 0.2 to 4% the size of the bacterial chromosome. Under most conditions of growth, plasmids are dispensable to their host cells. However, many plasmids contain genes that have a selective benefit in particular environments. For example, R plasmids render their host cells resistant to certain antibiotics, so in nature a cell containing such a plasmid can survive better in environments in which the antibiotic is present. Conjugation requires four activities: (1) interaction between specific donor and recipient cells, (2) sites on the plasmid that allow mobilization of the plasmid DNA, (3) transfer of the plasmid DNA into the recipient cell, and (4) re-formation of a functional plasmid in the recipient cell (Figure 6). Some plasmids can carry out all four steps of this process, but many plasmids lack the genes needed to carry out some subset of these processes. However, in many cases the conjugative functions Bacterial Genetics 55 donor d R-p~smkl ~Chromosom~ recipient ~.ffhromosome~ R-plasmid ll-cell contact mediated by pili donor recipient il ~ Mobilization occurs by nicking the DNA at Orff then rolling circle replication of R-plasmid DNA and transfer into recipient cell. donor recipient I Second strand of DNA is synthesized and DNA circularizes in recipient cell. After the-plasmid is completely transferred, cells dissociate. Both donor and recipient now have complete R-plasmid and can conjugate with other R- cells. donor donor ..... R-p~smid I Figure 6. Conjugation between a donor bacterium carrying an R-plasmid and a plasmid-free recipient strain.
- Kumar, Har Darshan(Authors)
- 2021(Publication Date)
- Daya Publishing House(Publisher)
Transformants included members of the genera Vibrio and Pseudomonas, as well as members of unknown genera. On the basis of many recent studies, it is concluded that transformation is quite important in the spread of genetic information through natural bacterial populations. Conjugation Conjugation in bacteria was first discovered in Escherichia coli by Lederberg and Tatum (1946), but the mechanisms involved in conjugation especially in the environment, are still not completely known. It is supposed to be primarily a plasmid–associated process in which conjugative plasmids carry transfer ( tra ) genes that not only mediate self–transfer but can also mobilize nonself–transmissible plasmids ( tra– ) and chromosomal genes to transfer unidirectionally from donor to recipient cell via cell contact. But another type of nonplasmid conjugative element, the conjugative transposon, has also been discovered (Franke and Clewell, 1981; Salyers et al ., 1995). Further, another type of conjugation, called retrotransfer, has been described (Mergeay et al ., 1987), in which there is reciprocal genetic exchange, that is, donor bacteria can also obtain genes from recipient bacteria via the movement of nonconjugative plasmids and chromosomal genes into cells containing some types of conjugative plasmids. Note that this direction of gene transfer is opposite to that in classical conjugation and mobilization (Yin and Stotzky, 1997). This ebook is exclusively for this university only. Cannot be resold/distributed. Mechanisms of Conjugation Self-Transfer of Conjugative Plasmids In Gram–negative bacteria, conjugation occurs in two stages. In the first, a specific cytoplasmic bridge is formed between the mating cells, whereby the plasmid–containing donor and the recipient contact each other through extracellular conjugative pili, which have been broadly divided into two morphological groups: (1) long flexible pili and (2) short rigid pili.
eBook - PDF- Michael Syvanen, Clarence I. Kado(Authors)
- 2001(Publication Date)
- Academic Press(Publisher)
In Chapter 6, Weld and Heinemann review protein transfers, a topic that has captured attention in recent years be-cause of its importance in pathogenic mecha-nisms. Protein transfer is probably also important in ensuring survival of transferred DNA in foreign cells. As is clear, bacteria have numerous and highly adapted mechanisms in place to facilitate the transfer of DNA from donor to recipient cells. These mechanisms do not respect species boundaries. The question as to whether or not these mech-anisms operate in natural populations is the subject of the remaining three chapters in this section. It has been known for many years that conjugal plasmid transfer occurs among bac-teria in hospitals, farms and natural environ-ments. Along these lines, Madsen in Chapter 4 has an interesting story that documents the emergence of a plasmid that makes enzymes which degrade coal tars and has spread among different bacterial species in a toxic waste dump. Chapters 7 (by Day) and 8 (by Miller and Ripp) show evidence that the DNA transfer mecha-nisms of transformation and bacterial virus transduction operate efficiently in natural environments. 1 This Page Intentionally Left Blank C H A P T E R 1 Recent History of Trans-kingdom Conjugation Gayle C. Ferguson and Jack A. Heinemann Conjugation is a mechanism of horizontal gene transfer (HGT) first observed between bacteria. The conjugative mechanism appears to be ana-logous, and sometimes homologous, to other means of transferring genes from bacteria to possibly members of every biological kingdom. As such, conjugative mechanisms of DNA transfer are necessary for a host of spectacular phenotypes such as symbiosis, virulence and antibiotic resistance. The conjugative mecha-nism is also related to the means of translocating and transferring proteins from bacteria to other species. Thus, this nearly generic form of macromolecular transport may move genes and other molecules across species boundaries.
eBook - PDFBiofilms
Recent Advances in their Study and Control
- L V Evans(Author)
- 2000(Publication Date)
- CRC Press(Publisher)
6 Plasmid Transfer between Bacteria in Biofilms Mark L.Angles and Amanda E.Goodman Biofilms are environments of high microbial cell density where cell- cell contact is likely. Such conditions create a favourable niche for the spread of self-transmissible as well as mobilisable plasmids among members of the bacterial communities. Studies have demonstrated plasmid transfer among bacteria in a wide range of biofilm habitats, including the surfaces of stones in a river, the air-water interface, surfaces in soil and water microcosms, plant surfaces and insect as well as animal intestinal surfaces. KEY WORDS: conjugative plasmids, biofilms, plasmid transfer WHAT ARE PLASMIDS AND HOW DO THEY SPREAD THROUGH BACTERIAL POPULATIONS? Plasmids are circular pieces of DNA found almost ubiquitously in bacteria. Plasmids often carry genes which are favourable for the survival of bacteria in adverse environments, where selection pressure would increase the likelihood of their transfer, for example catabolic genes and antibiotic and heavy metal resistance genes (Stotzky and Babich, 1986; Trevors et al., 1987; Sayler et al., 1990). Conjugation is the direct transfer of genetic information, in the form of plasmids, between bacterial cells and is dependent on cell-to-cell contact. Self-transmissible (conjugative) plasmids can be readily isolated from bacteria in most environments (Sizemore and Colwell, 1977; Stotzky and Babich, 1986; Hermansson et al., 1987; Trevors et al., 1987; Fredrickson et al., 1988; Genthner et al., 1988; Fry and Day, 1990; Sayler et al., 1990; Dahlberg et al., 1998a; 1998b). Conjugal transfer of plasmids occurs between closely related bacterial strains as well as between diverse genera (for reviews see Stotzky and Babich, 1986; Trevors et al., 1987; Trevors and Oddie, 1986; Ippen-Ihler, 1989; Mazodier and Davies, 1991; Veal et al., 1992). Cells into which plasmids have self-transferred (i.e. conjugated) are called transconjugants.
- Tina M. Henkin, Joseph E. Peters(Authors)
- 2020(Publication Date)
- ASM Press(Publisher)
Escherichia coli with others resulted in strains that were genetically unlike either of the originals. As noted in the introduction, Lederberg and Tatum suspected that bacteria of the two strains exchanged DNA—that is, two parental strains mated to produce progeny unlike themselves but with characteristics of both parents. At that time, however, plasmids were unknown, and it was not until later that the basis for the mating was understood.The plasmid that Lederberg and Tatum were unknowingly working with—called the fertility plasmid, or F plasmid —has been the focus of much of the research about the process of Bacterial Conjugation. The central role conjugation systems played in the development of bacterial genetics contrasts with the more notorious role these plasmids play as vectors for the spread of antibiotic resistance between bacteria. While historically studied with plasmids, conjugation is now also known to be used very frequently in the transfer of elements that are not maintained separately from the chromosome but instead are integrated into the chromosome and are therefore called integrating conjugative elements (ICEs). Their integration into the host chromosome typically uses the same type of recombination that is used for the integration of many bacteriophages. The same families of conjugation systems are found in ICEs and conjugal plasmids, indicating that over evolutionary time the same systems are alternatively utilized for intra- and extrachromosomal lifestyles (see Cury et al ., Suggested Reading).Overview
During plasmid conjugation, the two strands of an element separate in a process resembling rolling-circle replication (see “Mechanism of DNA Transfer during Conjugation in Proteobacteria” below), and one strand moves from the bacterium that originally contained the plasmid—thedonor—into a recipient bacterium. The two single strands serve as templates for DNA replication concurrent with the process of DNA transfer to yield double-stranded DNA molecules in both the donor cell and the recipient cell. A recipient cell that has received DNA as a result of conjugation is called a transconjugant . A simplified view of conjugation is shown in Figure 5.1
eBook - ePubAdvanced Molecular Biology
A Concise Reference
- Richard Twyman(Author)
- 2018(Publication Date)
- Garland Science(Publisher)
Chapter 10Gene Transfer in Bacteria
Fundamental concepts and definitions- Gene transfer describes the introduction of genetic information into a cell from an exogenous source (ultimately, another cell). This process occurs naturally in both bacteria and eukaryotes, and may be termed horizontal or lateral genetic transmission to distinguish it from the transmission of genetic information from parent to offspring, which is vertical genetic transmission.
- Intraspecific gene transfer facilitates genetic mixing in asexual species and thus mimics the effects of sexual reproduction. Such parasexual exchange mechanisms have been exploited to map prokaryote genomes analogously to meiotic mapping (q.v.) in eukaryotes. Interspecific gene transfer can also occur, and is a natural mechanism of transgenesis (q.v.). Interspecific gene transfer is an important evolutionary process and has been responsible for some of the most fundamental evolutionary events (e.g. the endosymbiont origin of eukaryotic organelles) as well as facilitating specific interactions between bacteria and eukaryotes (e.g. tumor-induction by Agrobacterium tumorfaciens; q.v. Ti plasmid).
- The source of the transferred information is the donor and the genetic information transferred is the exogenote (exogenous genome, usually only a fragment of the donor genome). The target of the gene transfer, the recipient, possesses the endogenote (endogenous genome). If the exogenote is homologous to part of the endogenote, gene transfer will make the recipient cell partially diploid (a merozygote), in which case recombination can occur, which may involve allele replacement (marker exchange).
- There are four major mechanisms of gene transfer in bacteria: cell fusion, conjugation, transformation and transduction (Table 10.1 ).
eBook - PDF- Elmer H. Marth, James Steele, Elmer H. Marth, James Steele(Authors)
- 2001(Publication Date)
- CRC Press(Publisher)
laetis (Gasson, 1980; Okamoto et aI., 1983). Reports of interspecific and even interge-neric gene exchange among LAB followed (Cocconcelli et aI., 1986; Iwata et aI., 1986; Kanatani et aI., 1990; Smith, 1985), and the method has even seen limited application for strain improvement (Stoianova et aI., 1988). Overall, how-ever, interest in protoplast fusion technology has never been high because of the need to establish stringent protoplast formation and regeneration conditions for individual strains (Alfoldi, 1982). Nonetheless, protoplast fusion may still be a useful method to combine desirable traits (e.g., production of inhibitors or phage defense systems) from distinct strains, species, or even genera into a single novel bacterium. C. Conjugation Conjugation is a natural form of gene transfer in bacteria that requires physical contact between viable donor and recipient cells. Because it facilitates horizontal gene exchange among populations of both related and unrelated microorganisms, conjugation has weighty implications on bacterial evolution and adaptation (Arber, 2000; Firth et aI., 1996). Genes required for conjugative transfer are typi-cally located on self-transmissible plasmids and conjugative transposons, but transfer of nonconjugative plasmids can also be effected via processes termed donation and conduction (Steele and McKay, 1989). The former process applies to nonconjugative plasmids that possess a specific sequence, called the origin of transfer (oriT), that is required for DNA mobilization. Transfer of these plasmids relies only upon trans-acting gene products from a conjugative element and not on cointegrate formation between the nonconjugative and conjugative elements. In contrast, plasmid transfer by conduction does require cointegration, because the nonconjugative molecule lacks a functional oriT.
eBook - PDF- D. Peter Snustad, Michael J. Simmons(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
If, by contrast, two genes are closely linked, they may be carried on a single molecule of transforming DNA, and double transformants may be formed at a high frequency. The frequency with which two genetic markers are cotransformed can thus be used to estimate how far apart they are on the host chromosome. CONJUGATION Transformation does not occur in E. coli—the most intensely studied bacterial species—under natural conditions. Thus, we could ask if there is any kind of gene transfer between E. coli cells. The answer to this question is “yes.” In 1946, Joshua Lederberg and Edward Tatum dis- covered that E. coli cells transfer genes by conjugation. Their important discovery is discussed further in A Milestone in Genetics: Conjugation in Escherichia coli on the Student Companion site. Conjugation has proven to be an important method of genetic mapping in bacterial species where it occurs, and it is an invaluable tool in genetic research. During conjugation, DNA is transferred from a donor cell to a recipient cell through a specialized intercellular conjugation channel or bridge, which forms between them (◾ Figure 8.13). Note that the donor and recipient cells are in direct contact during conjugation; the separation observed in Figure 8.13 is the result of stretching forces during preparation for microscopy. Additional proteins that mediate recombination A competent bacterium can bind exogenous DNA and transport it into the cell. The single strand of donor DNA is integrated into the chromosome of the recipient cell producing a DNA heteroduplex with different alleles in the two strands. Single-strand DNA-binding protein a – Competent bacterium Transformed bacterium Replaced recipient strand will be degraded. 1 S T E P 2 S T E P 3 S T E P Recipient chromosome a – a – Heteroduplex DNA from donor cell DNA receptor and translocation complex a + Exogenous DNA is bound to the receptor complex by the competence proteins ComEA and ComG.
eBook - PDFMicrobiology
Principles and Explorations
- Jacquelyn G. Black, Laura J. Black(Authors)
- 2018(Publication Date)
- Wiley(Publisher)
DONOR CELL RECIPIENT CELL F + F – F plasmid Bacterial chromosome Conjugation bridge F + F + © John Wiley and Sons, Inc. Conjugation 215 In the transfer of F + and F′ plasmids, as in all other transfers during conjugation, the donor cell retains all the genes it had prior to the transfer, including copies of the F plasmid. Single-stranded DNA is transferred, and both donor and recipient cells synthesize a complementary strand for any single-stranded DNA they contain. The results of conjugation with F + , Hfr, and F′ trans- fers are summarized in Table 9.1. The Significance of Conjugation Like other mechanisms for gene transfer, conjugation is significant because it contributes to genetic variation. Larger amounts of DNA are transferred in conjugation than in other transfers, so conjugation is especially impor- tant in increasing genetic diversity. In fact, conjugation may represent an evolutionary stage between the asexual pro- cesses of transduction and transformation and the actual fusion of whole cells (the gametes) that occur during sex- ual reproduction in eukaryotes. For the microbial geneti- cist, conjugation is of special significance because precise linear transfer of genes is useful in chromosome mapping. Conjugation also allows for replacement of genes that were damaged by mutation, which gene repair mechanisms failed to repair. After exposure to ultraviolet light, which causes mutations, some Archaea will quickly sprout pili which adhere to others of the same species and make con- jugation more probable, thereby replacing damaged genes. Plasmids that are self-transmissible—that is, have genes for the formation of an F pilus—can sometimes FIGURE 9.9 High-frequency recombinations. (a) Conversion of F + cells to the Hfr condition. Hfr cells arise from F + cells when their F plasmid is incorporated into a bacterial chromosome at one of several possible sites.
- E.M.H. Wellington, J.D. van Elsas(Authors)
- 2013(Publication Date)
- Pergamon(Publisher)
Although information on the occurrence of these mechanisms in the natural environment is still rather limited, there seems to be little doubt that all play a role in the dissemination of genes among bacterial communities in natural settings (see also Chapters 4, 12 and 14). For example, transformation has been suggested to be the underlying mechanism involved in the evolution of B. subtilis strains added to sterile soil (Graham & Istock, 1978). Also, Pseudomonas stützen has recently been shown to take up and express genetic material in marine sediment microcosms (Stewart & Sinigalliano, 1990). Transduc-tion has also been shown to play a role in gene transfer in natural habitats. For instance, Saye et al. (1987) revealed transduction of Pseudomonas aeruginosa in a freshwater environment. In addition, E. coli cells added to soil were shown to be transduced by an introduced bacteriophage (Germida & Khachatourians, 1988; Zeph et al., 1988) Conjugative gene transfer has also been shown to take place in natural aquatic and terrestrial environments, and recently-compiled information on this (Trevors et al., 1987; Stotzky, 1989; Saye & Miller, 1989) affirmed conjugation is an ecologically-important genetic transfer process. According to current understanding, transformation and transduction are transfer mechanisms with limited ecological potential. Firstly, only a limited number of bacterial species (e.g. Bacillus, P. stutzen) is known to be naturally transformable. Secondly, there are restrictions to the size and type of the DNA that can be successfully taken up by transformable cells. Homology is often required for stable maintenance of introduced DNA. However, much of our current knowledge is probably biased towards common laboratory strains, and transformability of many strains in the environment under environmental conditions is unknown.
eBook - PDF- Papita H Gourkhede(Author)
- 2023(Publication Date)
- Arcler Press(Publisher)
A white-eyed male fruit fly (Drosophila melanogaster) mutation discovered in a culture of red-eyed fruit flies led to the discovery of the chromosomal basis of inheritance, the underlying explanation that evaded Mendel. Nowadays, biotechnology allows people to directly create heritable alterations in cells to produce new protein products. Many methods exist for bacterial genetic exchange. The receptive bacteria absorb extracellular donor DNA during transformation. Donor DNA packed in a bacteriophage infects the recipient bacteria during transduction. Through mating, the donor bacteria transmit DNA to the receiver. The rearranging of Genetic Information Transfer 65 donor and recipient genomes to generate new, hybrid genomes are known as recombination. Transposons are movable DNA segments that transfer from one genome to the next. The biological importance of sexuality in microorganisms is that it increases the likelihood that uncommon, separate mutations may coexist in a single bacterium and also be exposed to natural selection. Microbe- microbe genetic interactions allow their genomes to develop considerably faster than mutation itself. The fast appearance and spread of antibiotic resistance plasmids, flagellar phase variation in Salmonella, and antigenic variation of surface antigens in Neisseria and Borrelia are examples of medically significant occurrences involving genetic information exchanges or genomic rearrangements. Reproductive activities in bacteria entail the transfer of genetic information from a source to a recipient, which results either in source allele replacement or the introduction of donor genetic material to the recipient genome. Transformation, transduction, and conjugation are sexual processes in which donor DNA is introduced into recipient bacteria in various ways.
eBook - PDF- Nancy Trun, Janine Trempy(Authors)
- 2009(Publication Date)
- Wiley-Blackwell(Publisher)
1998. DNA transfer from Agrobacterium to plant cells in crown gall tumor disease. Subcellular Biochemistry, 29: 343–63. Kado, C.I. 1998. Origin and evolution of plasmids. Antonie van Leeuwenhoek, 73: 117–26. Kornberg, A. and Baker, T. 1992. Plasmids and organelles. In DNA replication. pp. 637–87. New York: W.H. Freeman. Novick, R.P. 1980. Plasmids. Scientific American, December: 102. Chapter 10 Conjugation Conjugation is the process of moving DNA from one cell to another using a specific type of plasmid to mediate the transfer of DNA and requiring direct cell-to-cell con- tact. This process was first discovered by Lederberg and Tatum in 1946. Not all E. coli strains and not all bacteria are capable of conjugation. To be able to carry out this process, a bacterium must carry an F factor or an R factor and be physically mixed with a bacterium that does not contain one of these plasmids. Through a series of steps, the F or R factor can move itself and, in some instances the chromosome, from one cell to another. The F factor The F factor or F for short is a 100 kilobase pair plasmid (Fig. 10.1). The name F factor stands for fertility factor and was given to the plasmid based on the ability of F to me- diate conjugation. The F factor was described genetically long before plasmids were discovered, which is why it was first called a factor and not a plasmid. For historical reasons, the name F factor is still used. The F factor contains origins for replicating the plasmid during cell growth (RepF1A and RepF1B). Because it is present at one to two copies per cell, F has a partition system to ensure that both daughter cells inherit a copy (see Chapter 9 for details). F contains an origin of transfer or oriT where the transfer of the F factor DNA from one cell to another begins. oriT is physically separated from the origins used to replicate the F DNA during normal growth.
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