DNA photolyase homologs are the major UV resistance factors in the cyanobacterium Synechocystis sp PCC 6803

In this study, the unicellular photosynthetic cyanobacterium Synechocystis sp. PCC 6803 was used as a model phototroph to study the contribution of en zymatic photoreactivation to the overall protection against UV irradiation. We have isolated genes encoding two DNA photolyase homologs, phrA and phrB , from Synechocystis 6803. phrA encodes an 8-hydroxy-5-deazariboflavin (HDF ) type, Class I DNA photolyase. By complementing a photolyase-deficient mut ant strain of Escherichia coli, we demonstrated that PhrA is a DNA photolya se. Analysis of a phi A knockout mutant strain suggested that this gene is responsible for the majority of the observed UV resistance in Synechocystis 6803. Similar studies on phrB demonstrated that it also contributes to pho toreactivation, but to a much lesser degree. Based on these findings, we co nclude that enzymatic photoreactivation is the primary process used for rep airing UV-induced damage in Synechocystis 6803.