Regulation of the rulAB mutagenic DNA repair operon of Pseudomonas syringae by UV-B (290 to 320 nanometers) radiation and analysis of rulAB-mediated mutability in vitro and in planta

The effects of the rulAB operon of Pseudomonas syringae on mutagenic DNA re pair and the transcriptional regulation of rulAB following irradiation with UV-B wavelengths were determined. For a rulB::Km insertional mutant constr ucted in P. syringae pv. syringae B86-17, sensitivity to UV-B irradiation i ncreased and UV mutability decreased by 12- to 14-fold. rulAB-induced UV mu tability was also tracked in phyllosphere populations of B86-17 for up to 5 days following plant inoculation. UV mutability to rifampin resistance (Ri f(r)) was detected at all sampling points at levels which were significantl y greater than in nonirradiated controls. In P. aeruginosa PAO1, the cloned rulAB determinant on pJJK17 conferred a 30-fold increase in survival and a 200-fold increase in mutability following a UV-B dose of 1,900 J m(-2). In comparative studies using defined genetic constructs, we determined that r ulAB restored mutability to the Escherichia coli umuDC deletion mutant RW12 0 at a level between those of its homologs mucAB and umuDC. Analyses using a rulAB::inaZ transcriptional fusion in Pseudomonas fluorescens Pf5 showed that rulAB was rapidly induced after UV-B irradiation, with expression leve ls peaking at 4 h. At the highest W-B dose administered, transcriptional ac tivity of the rulAB promoter was elevated as much as 261-fold compared to t hat of a nonirradiated control. The importance of rulAB for survival of P. syringae in its phyllosphere habitat, coupled with its wide distribution am ong a broad range of P. syringae genotypes, suggests that this determinant would be appropriate for continued investigations into the ecological ramif ications of mutagenic DNA repair.