Inactivation of the spirochete recA gene results in a mutant with low viability and irregular nucleoid morphology

Recently, we have shown the first evidence for allelic exchange in Leptospira spp. By, using the same methodology, the cloned recA of Leptospira biflexa was interrupted by a kanamycin resistance cassette, and the mutated allele was then introduced into the L. biflexa chromosome by homologous recombination. The recA double-crossover mutant showed poor growth in liquid media and was considerably more sensitive to DNA-damaging agents such as mitomycin C and UV light than the wild-type strain. The efficiency of plating of the recA mutant was about 10% of that of the parent strain. In addition, microscopic observation of the L. biflexa recA mutant showed cells that are more elongated than those of the wild-type strain. Fluorescent microscopy of stained cells of the L. biflexa wild-type strain revealed that chromosomal DNA is distributed throughout most of the length of the cell. In contrast, the recA mutant showed aberrant nucleoid morphologies, i.e., DNA is condensed at the midcell. Our data indicate that L. biflexa RecA plays a major role in ensuring cell viability via mechanisms such as DNA repair and, indirectly, active chromosome partitioning.