Distinctive features of homologous recombination in an 'old' microorganism, Lactococcus lactis

Homologous recombination is needed to assure faithful inheritance of DNA ma terial, especially under stress conditions. The same enzymes that repair br oken chromosomes via recombination also generate biodiversity. Their activi ties may result in intrachromosomal rearrangements, assimilation of foreign DNA, or a combination of these events. It is generally supposed that homol ogous recombination systems are conserved, and function the same way everyw here as they do in Escherichia coli, the accepted paradigm. Studies in an ' older' microorganism, the Gram-positive bacterium of the low GC branch Lact ococcus lactis, confirm that many enzymes are conserved across species line s. However, the main components of the double strand break (DSB) repair sys tem, an exonuclease/helicase (Exo/hel) and a short DNA modulator sequence C hi, differ markedly between bacteria, especially when compared to the Gram- negative analogues. Based on our studies, a model is proposed for the funct ioning of the two-subunit Exo/hel of L. lactis and other Gram-positive bact eria, which differs from that of the three-subunit E. coli enzyme. The diff erences between bacterial DSB repair systems may underlie a selection for d iversity when dealing with DSB. These and other features of homologous reco mbination in L. lactis are discussed, (C) 2001 Editions scientifiques et me dicales Elsevier SAS.