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 enzymatic 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 mutant strain of Escherichia coli, we demonstrated that PhrA is a DNA photolyase. Analysis of a phrA 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 photoreactivation, but to a much lesser degree. Based on these findings, we conclude that enzymatic photoreactivation is the primary process used for repairing UV-induced damage in Synechocystis 6803.