A partial photoreactivation defect phenotype is not due to unrepaired ultraviolet-induced pyrimidine dimers in ultraviolet-sensitive mutants of <i>Neurospora crassa</i>

Photoreactivation is a mechanism in which photolyase directly repairs either cyclobutane pyrimidine dimers (CPDs) or (6-4) photoproducts [(6-4) PPs] caused by ultraviolet (UV) light. In the filamentous fungus Neurospora crassa, some UV-sensitive mutants such as mus-44 have been reported to exhibit a partial photoreactivation defect (PPD) phenotype, but its mechanism has not been elucidated for a long time. In this study, the N. crassa CPD photolyase PHR was overexpressed in the Δmus-44 strain, but photoreactivation ability was not increased. Furthermore, Escherichia coli CPD photolyase or Arabidopsis thaliana (6-4) PP photolyase was also introduced into Δmus-44; however, the PPD phenotype was not complemented. These results suggested that the PPD phenotype in N. crassa is not caused by residual unrepaired pyrimidine dimers, which are the main type of DNA damage caused by UV irradiation. Finally, we revealed that Δmus-44, but not the Δmus-43 strain, which does not show the PPD phenotype, displayed higher sensitivity with increasing dose rate of UV. Moreover, Δmus-44 was also sensitive to an interstrand crosslinking agent. This indicates that the high dose of UV in our experimental condition induces DNA damage other than pyrimidine dimers, and that such damage is a likely cause of the PPD phenotype.