Partial Conversion of an Animal-Like Cryptochrome into a Plant Cryptochrome

Cryptochromes act as flavin-binding photoreceptors in many organisms. The green alga Chlamydomonas reinhardtii contains both a plant cryptochrome (pCRY) and an animal-like cryptochrome (aCRY) with very distinct photochemistry. pCRY functions as a blue light receptor, whereas dual-function aCRY acts as a (6-4) photolyase and as a photoreceptor up to 680 nm. aCRY additionally uses 8-hydroxy-5-deazaflavin (8-HDF, F0) as a light-harvesting pigment. The proton donor to flavin in pCRY, an aspartic acid, is replaced by an asparagine in aCRY. Here, the effects of the exchange in aCRY-N395D are studied with and without 8-HDF using nanosecond time-resolved UV-vis and FTIR difference spectroscopy. We show that the exchange of a single amino acid transforms both the photochemistry and the conformational response, even to the level of functionality, by slower photoactivation for DNA repair. Proton transfer from D395 to flavin and hypsochromic shifts occur in aCRY-N395D as in pCRY but with ultrafast kinetics. The flavin neutral radical is formed before 100 ns as opposed to microseconds in pCRY and milliseconds in aCRY. Hallmarks of conformational changes of plant cryptochromes are initiated in aCRY-N395D highlighting the importance of aspartate for signaling. These insights strongly improve our understanding of the differentiation of protein functions within the cryptochrome/photolyase superfamily.