Conjugation is one of several mechanisms that bacteria use to transfer DNA, and hence new genetic information, between two cells. The other primary mechanisms are transformation, in which free DNA is transported across the cell membrane, and transduction, in which DNA is carried into the recipient cell by a bacterial virus.
The Role of Plasmids
Conjugation is about as close as single cells come to engaging in sex, and some of the terminology used to describe the process reflects that similarity. Conjugation, or mating, is a process of genetic transfer that requires cell-to-cell contact. The genetic instructions for conjugation are encoded on a double-stranded, circular piece of DNA. The circular DNA exists in the bacterial cell entirely separate from the much larger bacterial chromosome. Scientists refer to this specialized, extrachromosomal piece of DNA as a conjugative plasmid or a "fertility factor." Cells that possess it are donor or "male" cells, and those that lack a conjugative plasmid are recipient or "female" cells.
There are multiple genes involved in the process of conjugation. Some of the genes code for a surface structure found on donor cells, the sex pilus. This is a threadlike tube made of protein. The sex pilus recognizes a specific attachment site on a recipient cell. When the donor cell comes near a recipient, the sex pilus attaches to the specific site and begins to retract, pulling the two cells together. This is a bit like throwing out a fishing line, hooking a fish, and pulling it into shore. The fishing analogy ends here, however. As the two cells draw close, their connection stabilizes and their outer membranes fuse together to allow the transfer of DNA from one cell to the other.
Only one of the two strands of DNA making up the plasmid passes through the fused membranes into the recipient cell. Thus DNA synthesis must occur in both donor and recipient to replace the missing strand in each. The genes encoding the enzymes responsible for this part of the conjugative process are also found on the plasmid. Once passage and synthesis are successfully completed, both donor and recipient cells contain a whole double-stranded, circular, conjugative plasmid. Thus there are now two donor cells when before there was only one. This process is so efficient that it can quickly change an entire population to donor cells. Some types of conjugative plasmids are transferred only between cells of the same species. Other types can be transferred across species; scientists call them promiscuous plasmids.
Large-Scale Gene Transfer
One of the two scientists who first described conjugation, Joshua Lederberg, ultimately won the Nobel Prize in medicine in 1958 for his discoveries concerning the organization of genetic material in bacteria. In 1946 Lederberg and his colleague E. L. Tatum set out to determine whether a sexual process might occur in bacteria. The bacterial species he used in the experiments was Escherichia coli. This was fortuitous, as it turned out, because E. coli often contains a special kind of conjugative plasmid that has the ability to insert itself into the cell's chromosome. Once this happens, the donor cell can transfer to a recipient not only plasmid genes but also large numbers of chromosomal genes.
Lederberg worked with two different nutritional mutants of E. coli. One strain required biotin and methionine to grow; the other strain required threonine and leucine. Lederberg mixed the two strains together and then attempted to grow them without supplying any of the four nutrients. His hypothesis was that any cell able to grow without the four nutrients would have all four genes intact, and would thus have received the functioning genes from the other strain and incorporated them into its chromosome. The incorporation of the genes in this manner is called genetic recombination.
As he predicted, Lederberg's experiment yielded cells that did not require any of the nutrients to grow. In a second set of experiments, Lederberg showed that cell-to-cell contact was necessary for genetic recombination to occur. Over several years, he and other scientists discovered the mechanics of the entire process that we now call conjugation.
From the human perspective, one of the significant consequences of a bacterium's ability to pass genetic information along to other cells via conjugation is its link to the widespread incidence of antibiotic resistance. The genes that encode for resistance to a variety of antibiotics like penicillin and tetracycline are commonly found on plasmids. When a population of susceptible bacteria is exposed to a given antibiotic, most of them will be killed. However, if the population contains cells with conjugative plasmids bearing the genes for resistance, they can rapidly spread the trait throughout the population. These plasmids are large and are often promiscuous, so that transfer of antibiotic resistance genes need not be restricted to cells of like species. In some cases, this has resulted in disease-causing bacteria that are resistant to almost every antibiotic available. For instance, antibiotic resistant tuberculosis bacteria are a significant public health threat in some metropolitan areas.
Cynthia A. Needham
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Conjugation is a mechanism whereby a bacterium can transfer genetic material to an adjacent bacterium. The genetic transfer requires contact between the two bacteria . This contact is mediated by the bacterial appendage called a pilus.
Conjugation allows bacteria to increase their genetic diversity. Thus, an advantageous genetic trait present in a bacterium is capable of transfer to other bacteria. Without conjugation, the normal bacterial division process does not allow for the sharing of genetic information and, except for mutations that occur, does not allow for the development of genetic diversity.
A pilus is a hollow tube constructed of a particular protein. One end is anchored to the surface of a bacterium. The other end is capable of binding to specific proteins on the surface of another bacterium. A pilus can then act as a portal from the cytoplasm of one bacterium to the cytoplasm of the other bacterium. How the underlying membrane layers form channels to the bacterial cytoplasm is still unclear, although channel formation may involve what is termed a mating pair formation (mpf) apparatus on the bacterial surface.
Nonetheless, once a channel has been formed, transfer of deoxyribonucleic acid (DNA ) from one bacterium (the donor) to the other bacterium (the recipient) can occur.
Conjugation requires a set of F (fertility) genes. Transfer of DNA from the genome of a bacterium can occur if the F set of genes is integrated in the bacterial chromosome. These F genes enter the pilus and literally drag the trailing genome along behind. Often the pilus will break before the transfer of the complete genome can occur. Thus, genes that are located in the vicinity of the F genes will tend to be successfully transferred in conjugation more often than genes located far away from the F genes. This process was originally discovered in Escherichia coli . Strains that exhibit a higher than usual tendency to transfer genomic DNA are known as High Frequency of Recombination (Hfr) strains.
Conjugation also involves transfer of DNA that is located on a plasmid. A plasmid that contains the F genes is called the F episome or F plasmid. Other genes on the episome will be transferred very efficiently, since the entire episome can typically be transferred before conjugation is terminated by pilus breakage. If one of the genes codes for a disease causing factor or antibiotic resistance determinant, then episomal conjugation can be a powerful means of spreading the genetic trait through a bacterial population. Indeed, conjugation is the principle means by which bacterial antibiotic resistance is spread.
Finally, conjugation can involve the transfer of only a plasmid containing the F genes. This type of conjugation is also an efficient means of spreading genetic information to other bacteria. In this case, as more bacteria acquire the F genes, the proportion of the population that is capable of genetic transfer via conjugation increases.
Joshua Lederberg discovered the process of conjugation in 1945. He experimented with so-called nutritional mutants (bacteria that required the addition of a specific nutrient to the growth medium). By incubating the nutritional mutants in the presence of bacteria that did not require the nutrient to be added, Lederberg demonstrated that the mutation could be eliminated. Subsequently, another bacteriologist, William Hayes, demonstrated that the acquisition of genetic information occurred in a one-way manner (e.g., information was passing from one bacterium into another), and that the basis for the information transfer was genetic (i.e., mutants were isolated in which the transfer did not occur).
Another landmark experiment in microbiology also centered on conjugation. This experiment is known as the interrupted mating experiment (or blender experiment, since a common kitchen blender was used). Donor and recipient bacteria were mixed together and left to allow conjugation to begin. Then, at various times, the population was vigorously blended. This sheared off the pili that were connected the conjugating bacteria, interrupting the mating process. By analyzing the recipient bacteria for the presence of known genes that has been transferred, the speed of conjugation could be measured.
Conjugation has been exploited in the biotechnology era to permit the transfer of desired genetic information. A target gene can be inserted into the donor bacterial DNA near the F genes. Or, an F plasmid can be constructed in the laboratory and then inserted into a bacterial strain that will function as the donor. When conjugation occurs, bacteria in the recipient population will acquire the target gene.
See also Evolution and evolutionary mechanisms; Laboratory techniques in microbiology
1. The fusion of two reproductive cells, particularly when these are both the same size (see isogamy).
2. A form of sexual reproduction seen in some algae (e.g. Spirogyra), some bacteria (e.g. Escherichia coli), and ciliate protozoans. Two individuals are united by a tube formed by outgrowths from one or both of the cells. Genetic material from one cell (designated the male) then passes through the tube to unite with that in the other (female) cell. In bacteria conjugation is initiated and directed by sex factors.