RecBCD and RecF Pathways

Homologous, or general, recombination is a crucial biological process that involves the paring and transfer of strands between DNA molecules that share a region of significant sequence homology. After its discovery in 1946 by Lederberg and Tatum (48), homologous recombination in bacteria was associated with the sexual process of conjugation and was viewed as an evolutionary mechanism both for shuffling the genome and for spreading favorable alleles. However, more recently, a more immediate function of homologous recombination has been recognized: namely, it is a mechanism for the maintenance of chromosomal integrity that acts to repair DNA lesions, both doublestrand DNA breaks and single-strand DNA gaps, generated during the course of DNA replication. The intimate connection between the processes of replication and recombination was initially appreciated in the life cycle of bacteriophage T4 (56) and then later recognized as an important determinant of viability in bacteria (39,45). In T4 phage, recombination is linked to replication to produce a high yield of phage DNA; in Escherichia coli, recombination is linked to replication to permit its completion when interrupted by DNA damage, and also to initiate DNA replication in the absence of origin function. This view of recombination as an integral part of efficient chromosome duplication also reconciled the high level of inviability (up to 95%) of recombination-deficient cells (12).