Mutagenic DNA repair potential inPseudomonasspp., and characterization of therulABPcoperon from the highly mutable strainPseudomonas cichorii302959

Abstract

We assessed the tolerance to ultraviolet B (UVB; 290-320 nm) radiation and UVB-induced mutability in 28 Pseudomonas spp. and four Burkholderia cepacia strains. The UVB survival of 23 (72%) of the strains was elevated (>46% survival following irradiation with a 2250 J m-2dose), and 17 (53%) strains were defined as mutable by UVB. A mutagenic DNA repair determinant was cloned and characterized from the highly mutable strain P. cichorii 302959 and shown by sequence analysis to be an allele of rulAB, a mutagenic DNA repair determinant previously characterized from Pseudomonas syringae. Phylogenetic analyses of RulA- and RulB-related sequences indicated that the sequences identified in environmental bacteria shared a common ancestor with UmuDC-like sequences from enteric bacteria but were considerably diverged. The dynamics of UVB-induced mutability to nalidixic acid resistance (NalR) and rifampicin resistance (RifR) were studied in replicate populations of P. cichorii 302959 subjected to a daily UVB dose of 2250 J m-2for 14 consecutive days. While there was an initial spike in the frequency of NalRand RifRmutants recovered on Days 1 and 2 of two separate experiments, the frequencies were sharply reduced and then fluctuated throughout the duration of both experiments. These experimental results are intriguing because they point to the possibility that P. cichorii possesses additional mechanisms to curtail the induction of spontaneous mutants following repeated episodes of UVB irradiation.Key words: phyllosphere, UVB survival, UVB mutability, long-term mutability, translesion synthesis, Y family DNA polymerase.