This module looks at how the process of recombination applies to haploid organisms like bacteria. Specifically, we will examine how genetic material is transferred between bacteria, allowing recombination to occur.
Transfer of Genetic Material in Bacteria
It may seem somewhat surprising that bacteria can undergo recombination. After all, as was outlined in the module on recombination, the process requires two homologous DNA molecules, and bacteria have only one chromosome (and are therefore haploid). Bacteria, however, have mechanisms by which they can 'obtain' extra DNA, which creates opportunities for recombination to occur. The three main mechanisms by which bacteria acquire new DNA are transformation, conjugation, and transduction. Transformation involves acquisition of DNA from the environment, conjugation involves acquisition of DNA directly from another bacterium, and transduction involves acquisition of bacterial DNA via a bacteriophage intermediate. Each of these mechanisms will be examined in turn.
Transformation is the process by which bacteria pick up DNA from their environment. The DNA may come from a variety of sources, but most likely it is the remnants of DNA from dead bacterial cells. We have seen an example of transformation before, in the module on nucleic acids, in which avirulent Streptococcus pneumoniae became virulent by being exposed to heat-killed virulent cells.
In order to become successfully transformed, bacteria must be competent. This means that the bacteria are expressing the appropriate enzymes (the 'transformation machinery') required to transport the exogenous DNA into the cell. Therefore, the correct genes must be expressed in order to carry out transformation. Expression of these genes depends on the growth conditions: bacteria most likely to be competent are dividing rapidly, but nutrients in the environment are becoming limited. (For more on the control of gene expression, see the module on bacterial gene regulation.)
In transformation, a cell surface receptor binds to DNA in the environment. After binding, the DNA is transported across the membrane by the transformation machinery. As this occurs, one strand of the DNA is digested away by an exonuclease, so that the DNA that enters the cell is single stranded. This promotes recombination, as long as the DNA taken up is sufficiently homologous to the host DNA to allow recombination to occur. The recombination that occurs is one-way (non-reciprocal); unlike the exchange of strands diagrammed in the module on recombination, in this case the new DNA will simply replace a strand of the host DNA. The replaced segment of host DNA will be degraded. If the new DNA is of a different allelic nature than the host DNA, a gene conversion event can occur. This is what happened in the example mentioned above: the avirulent strain of S. pneumoniae had a mutation in a gene required for production of the bacterial capsule. Heat killing the virulent cells (which contained the wild-type capsule gene) caused the release of fragments of the dead cells' genomes. Some of the avirulent cells picked up a piece of DNA containing the wild-type capsule gene, and underwent gene conversion so that they were wild type for that gene, causing them to become virulent.
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