Remarkable resistance to UVB of the marine bacterium Photobacterium angustum explained by an unexpected role of photolyase

DNA damage and cell survival was assessed in the marine bacteria, Photobacterium angustum (GC% = 39.6) and Sphingopyxis alaskensis (GC% = 65.5) following UVB irradiation and recovery in the presence or absence of visible light. The extent of bipyrimidine photoproduct formation was analyzed by HPLC-MS/MS. S. alaskensis was chosen as a reference species since it was previously shown to be photoresistant. Interestingly, P. angustum exhibited an even higher level of survival to UVB irradiation than S. alaskensis. This higher photoresistance was associated with a decrease in the rate of formation of cyclobutane pyrimidine dimers (CPDs) at high UVB doses. Despite different distributions in UVB-induced lesions, the survival difference between the two marine bacteria could not be accounted for by qualitative differences in either photoreactivation or the rate of nucleotide excision repair of the photoproducts arising from the different bipyrimidine doublets (TT, CT, TC and CC). Dark repair was found to be much more efficient for P. angustum than S. alaskensis but the corresponding rate of photoproduct removal was lower than that observed at high UVB doses. We propose that the increased resistance of P. angustum under high UVB doses results from a UVB-induction of CPD photolyase(s) that may directly repair DNA damage and/or act indirectly by enhancing the rate of nucleotide excision repair.