Replication arrest is a major threat to growth at low temperature in Antarctic Pseudomonas syringae Lz4W

Chromosomal damage was detected previously in the recBCD mutants of the Antarctic bacterium Pseudomonas syringae Lz4W, which accumulated linear chromosomal DNA leading to cell death and growth inhibition at 4°C. RecBCD protein generally repairs DNA double-strand breaks by RecA-dependent homologous recombination pathway. Here we show that ΔrecA mutant of P. syringae is not cold-sensitive. Significantly, inactivation of additional DNA repair genes ruvAB rescued the cold-sensitive phenotype of ΔrecBCD mutant. The ΔrecA and ΔruvAB mutants were UV-sensitive as expected. We propose that, at low temperature DNA replication encounters barriers leading to frequent replication fork (RF) arrest and fork reversal. RuvAB binds to the reversed RFs (RRFs) having Holliday junction-like structures and resolves them upon association with RuvC nuclease to cause linearization of the chromosome, a threat to cell survival. RecBCD prevents this by degrading the RRFs, and facilitates replication re-initiation. This model is consistent with our observation that low temperature-induced DNA lesions do not evoke SOS response in P. syringae. Additional studies show that two other repair genes, radA (encoding a RecA paralogue) and recF are not involved in providing cold resistance to the Antarctic bacterium.