Homologous Recombination in ESCHERICHIA COLI: Dependence on Substrate Length and Homology

We studied the in vivo recombination between homologous DNA sequences cloned in phage lambda and a pBR322-derived plasmid by assaying for the formation of phage-plasmid cointegrates by a single (or an odd number of) reciprocal exchange. (1) Recombination proceeds by the RecBC pathway in wild-type cells and by low levels of a RecF-dependent pathway in recBC- cells. The RecE pathway appears not to generate phage-plasmid cointegrates. (2) Recombination is linearly dependent on the length of the homologous sequences. In both RecBC and RecF-dependent pathways there is a minimal length, called the minimal efficient processing segment (MEPS), below which recombination becomes inefficient. The length of MEPS is between 23-27 base pairs (bp) and between 44-90 bp for the RecBC- and RecF-dependent pathways, respectively. A model, based on overlapping MEPS, of the correlation of genetic length with physical length is presented. The bases for the different MEPS length of the two pathways are discussed in relationship to the enzymes specific to each pathway. (3) The RecBC and the RecF-dependent pathways are each very sensitive to substrate homology. In wild-type E. coli, reduction of homology from 100% to 90% decreases recombinant frequency over 40-fold. The homology dependence of the RecBC and RecF-dependent pathways are similar. This suggests that a component common to both, probably recA, is responsible for the recognition of homology.