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9.6: Conjugation

There are two other processes that can lead to horizontal gene transfer in bacteria: conjugation and transduction. In contrast to transformation, these processes “force” DNA into what may be a reluctant cell. In the process of conjugation, we can distinguish between two types of bacterial cells (of the same species). One contains a plasmid known as the sex factor (F) plasmid. These are known as a Hfr (high frequency recombination) cells. This plasmid contains the genes needed to transfer a copy of its DNA into a cell that lacks an F-plasmid, a so called F–cell. Occasionally, the F-plasmid can integrate (insert) itself into the host cell chromosome and when this happens, the F-plasmid mediated system can transfer host cell genes (in addition to plasmid genes) into an F–cell. To help make things a little simpler, we will refer to the Hfr cell as the DNA donor and F–cells as the DNA recipients.

To initiate conjugation, the Hfr cell makes a physical bridge to the F–cell. A break in the donor DNA initiates a process by which single stranded DNA is synthesized and moved into the recipient (F–) cell. The amount of DNA transported is determined largely by how long the transporter bridge remains intact. It takes about 100 minutes to transfer the entire donor chromosome from an Hfr to an F–cell. Once inside the F–cell, the DNA is integrated into the recipient’s chromosome, replacing the recipient’s versions of the genes transferred (through a process known as homologous recombination). Using Hfr strains with integrated F–plasmids carrying different alleles of various genes, and by controlling the duration of conjugation (separating the cells by placing them in a kitchen blender), experimenters were able to determined the order of genes along the chromosome. The result was the discovery that related organisms had the same genes arranged in the same order. The typical drawing of the circular bacterial chromosome is like a clock going from 0 to 100, with the genes placed in their respective positions, based on the time it takes to transfer them (in minutes). This is an example of synteny, that is the conservation of gene order along a chromosome266. We will return to synteny soon.

If the entire F-plasmid sequence is transferred, the original F–cell becomes an Hfr cell. If the Hfr cell loses the F-plasmid sequence, it reverts to a F–state. The end result of the conjugation process is similar to that obtained in sexual reproduction in eukaryotes (see below), namely the original F–cell now has a genome derived in part from itself and from the “donor” Hfr strain cell.

References

266 Synteny: http://en.wikipedia.org/wiki/Synteny

Contributors

  • Michael W. Klymkowsky (University of Colorado Boulder) and Melanie M. Cooper (Michigan State University) with significant contributions by Emina Begovic & some editorial assistance of Rebecca Klymkowsky.