Conjugation - Bacterial Reproduction

CONJUGATION

Conjugation is thought to have evolved through transduction, and relates to the generation of defective viral DNA. This can be transcribed to produce singular viral elements, which cannot assemble or lyse the host cell. Such DNA strands are known as plasmids. They are circular and can either be integrated into the main chromosome, in which case they are replicated along with the chromosome and passed to daughter cells, or they are separate from it and can replicate independently. The simplest form of plasmid is the F-factor (fertility factor); this can be transcribed at the cell membrane to generate an F-pilus within the cell envelope and cells containing an F factor are designated F⁺. The F-pilus is a hollow appendage that is capable of transferring DNA from one cell to another, through a process that is very similar to the injection of viral DNA into a cell during infection. In its simplest form an un-associated F-factor will simply transfer a copy to a recipient cell, and such a transfer process is known as conjugation. Integration with, and dissociation of, the F-factor with the chromosome occurs randomly. When it is in the integrated form, designated Hfr (high frequency of recombination), then not only can a copy of the plasmid DNA be transferred across the F-pilus but so also can a partial or complete copy of the donor chromosome. Subsequent recombination events incorporate the new DNA into the recipient chromosome.

Just as the excision of temperate viral DNA from the host chromosome could be inaccurate, and lead to additions and deletions from the sequence, so too can the F factor gather chromosomal DNA as the host cells change from Hfr to F ⁺. In such instances the plasmid that is formed will transfer not only itself but also this additional DNA into recipient cells. This is particularly significant because the un-associated plasmid can replicate autonomously from the chromosome to achieve a high copy number. It can also be transferred simultaneously to many recipient bacteria. If the transported DNA encoded a mechanism of antibiotic resistance it would not be difficult to imagine how whole populations could rapidly acquire the resistance characteristics.