The mechanisms of homologous recombination in bacteria (described in Chapter 1) are ancient and highly conserved. The basic requirement is two DNA sequences that share sequence identity over a minimum distance, typically at least 40 to 50 nucleotides (Shen and Huang, 1986; Watt et al., 1985). These identical sequences are brought together to create, and eventually resolve, a recombinant molecule, by the actions of enzymes such as RecA, RecBCD/RecF, and RuvABC, or their equivalents (Kowalczykowski et al., 1994). Homologous recombination serves several purposes in the cell. The most fundamental of these, according to current understanding, is to facilitate the completion of chromosome replication (Cox, 2001; Smith, 2001). Replication forks frequently stall or break, and homologous recombination can solve this problem using the sister homolog as a template (Kuzminov, 1995). A related problem is that after replication bacterial chromosomes are frequently entangled and require homologous recombination to disentangle them.
In addition to its housekeeping roles, homologous recombination can shuffle the order of genes in a genome by recombination between repetitive sequences. It can also facilitate the incorporation of foreign DNA, although this is also achieved by site-specific recombination (Ochman et al., 2000). Lateral DNA transfer in bacteria can alleviate the effects of Muller's ratchet (Andersson and Hughes, 1996; Muller, 1964), and provide bacteria with access to a very large gene pool potentially containing important innovative properties (Ochman et al., 2000).