Photoprotective Role of Photolyase-Interacting RAD23 and Its Pleiotropic Effect on the Insect-Pathogenic Fungus Beauveria bassiana

Abstract

RAD23 can repair yeast DNA lesions through nucleotide excision repair (NER), a mechanism that is dependent on proteasome activity and ubiquitin chains but different from photolyase-depending photorepair of UV-induced DNA damages. However, this accessory NER protein remains functionally unknown in filamentous fungi. In this study, orthologous RAD23 in Beauveria bassiana, an insect-pathogenic fungus that is a main source of fungal insecticides, was found to interact with the photolyase PHR2, enabling repair of DNA lesions by degradation of UVB-induced cytotoxic (6-4)-pyrimidine-pyrimidine photoproducts under visible light, and it hence plays an essential role in the photoreactivation of UVB-inactivated conidia but no role in reactivation of such conidia through NER in dark conditions. Fluorescence-labeled RAD23 was shown to normally localize in the cytoplasm, to migrate to vacuoles in the absence of carbon, nitrogen, or both, and to enter nuclei under various stresses, which include UVB, a harmful wavelength of sunlight. Deletion of the rad23 gene resulted in an 84% decrease in conidial UVB resistance, a 95% reduction in photoreactivation rate of UVB-inactivated conidia, and a drastic repression of phr2 A yeast two-hybrid assay revealed a positive RAD23-PHR2 interaction. Overexpression of phr2 in the Δrad23 mutant largely mitigated the severe defect of the Δrad23 mutant in photoreactivation. Also, the deletion mutant was severely compromised in radial growth, conidiation, conidial quality, virulence, multiple stress tolerance, and transcriptional expression of many phenotype-related genes. These findings unveil not only the pleiotropic effects of RAD23 in B. bassiana but also a novel RAD23-PHR2 interaction that is essential for the photoprotection of filamentous fungal cells from UVB damage.IMPORTANCE RAD23 is able to repair yeast DNA lesions through nucleotide excision in full darkness, a mechanism distinct from photolyase-dependent photorepair of UV-induced DNA damage but functionally unknown in filamentous fungi. Our study unveils that the RAD23 ortholog in a filamentous fungal insect pathogen varies in subcellular localization according to external cues, interacts with a photolyase required for photorepair of cytotoxic (6-4)-pyrimidine-pyrimidine photoproducts in UV-induced DNA lesions, and plays an essential role in conidial UVB resistance and reactivation of UVB-inactivated conidia under visible light rather than in the dark, as required for nucleotide excision repair. Loss-of-function mutations of RAD23 exert pleiotropic effects on radial growth, aerial conidiation, multiple stress responses, virulence, virulence-related cellular events, and phenotype-related gene expression. These findings highlight a novel mechanism underlying the photoreactivation of UVB-impaired fungal cells by RAD23 interacting with the photolyase, as well as its essentiality for filamentous fungal life.

Keywords: DNA damage repair; UV damage; biological control; entomopathogenic fungi; nucleotide excision repair protein; photolyase; photoreactivation.