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Abstract

The cryptochrome/photolyase family (CPF) is a group of highly conserved flavoproteins that harness sunlight to enable various biological processes including hotoreception, DNA damage repair and circadian clock entrainment. Photoreactivation, one of the principal DNA repair systems is catalysed by the flavoprotein enzymes called photolyases. These use light as a driving force to repair UV-induced DNA damage and are encountered in almost all prokaryotes and eukaryotes with the notable, strange exception of placental mammals. The Foulkes lab’s previous work has demonstrated that the Somalian cavefish, Phreatichthys andruzzii, which has evolved in a perpetually dark subterranean environment for millions of years, has greatly attenuated photoreactivation as the result of accumulating truncation mutations in the 6-4 and DASH photolyase genes. However, the CPD photolyase gene remains intact and encodes a protein that can still catalyze DNA repair. Is there selective pressure acting to maintain CPD photolyase function despite the complete absence of sunlight? Here I have used a comparative approach involving photolyase mutant lines generated in medaka as well as zebrafish and cavefish, to reveal that CPD photolyase confers increased cell survival and enhanced DNA repair capacity upon exposure of cells to oxidative stress. Furthermore, I demonstrate that light does not influence CPD photolyase-induced protection against ROS-induced mortality. Interestingly, in the absence of light, ROS can induce limited CPD production however, it remains unclear whether CPD photolyase may be able to catalyze the repair of this DNA damage under constant darkness. My results may account for why the CPD photolyase gene is conserved in the Somalian cavefish. Furthermore, this may provide clues as to how and why photolyase genes have been lost during placental mammal evolution. It has been documented that in various organisms, many CPF flavoproteins possess a bifunctional property, specifically being not only implicated in DNA repair but also serving as circadian clock components. In this study, I have shown that the loss of 6-4 photolyase function disrupts rhythmic expression of certain clock genes in a gene- and tissue-specific manner. Furthermore, I have shown that 6-4 photolyase participates in the transcription control of circadian clock genes via repression of transactivation of the CLOCK-BMAL complex occurring at E-box enhancer elements and via enhancing TEF transactivation at light-responsive D-box enhancer elements. However, the precise nature of the physical interaction between 6-4 photolyase and the CLOCK-BMAL heterodimer and the TEF protein remains unclear. My discovery provides new insight into the basis of the divergent function of CPF members in vertebrates. Furthermore, more generally the results obtained in this project reveal how the evolution of the CPF family of flavoproteins may have been shaped by adaptation to extreme environmental conditions.