Conjugation

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.

Antibiotic Resistance

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

Bibliography

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Snyder, Larry, and Wendy Champness. Molecular Genetics of Bacteria. Washington, DC: ASM Press, 1997.