Replication arrest is a major threat to growth at low temperature in A ntarctic P seudomonas syringae Lz 4 W

Summary Chromosomal damage was detected previously in the recBCD mutants of the A ntarctic bacterium P seudomonas syringae   Lz 4 W , 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 Δ recA mutant of P . syringae is not cold‐sensitive. Significantly, inactivation of additional DNA repair genes ruvAB rescued the cold‐sensitive phenotype of Δ recBCD mutant. The Δ recA and Δ ruvAB 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 H olliday 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 A ntarctic bacterium.