The role of cryptochromes in the sexual life cycle of Chlamydomonas reinhardtii

Cryptochromes are known as blue light sensory flavoproteins in plants, insects, fungi, and bacteria and as part of the circadian oscillator, the latter mainly in mammals. An animal-like cryptochrome (aCRY) from Chlamydomonas reinhardtii was recently characterized as a blue and red light photoreceptor that plays a crucial role in the regulation of light-dependent genes involved in chlorophyll and carotenoid biosynthesis, light-harvesting complex assembly, nitrogen metabolism, cell cycle control and the circadian clock. In this work, aCRY was found to be mainly a soluble protein, which is also attached to membranes to some extent. Soluble aCRY accumulates in vegetative cells and mature zygotes, but not in pregametes, gametes and early zygotes. The degradation of soluble aCRY in pregametes, gametes and early zygotes is processed by the proteasome pathway. In contrast to soluble aCRY, membrane-associated aCRY accumulates in vegetative cells, pregametes, gametes, and late zygotes, but not in early zygotes. Based on an available acry mutant, acry mutant strains of mating type plus and minus with similar genomic background as wild-type strains were generated by backcrossing. Also complemented strains of these mutants were generated. Based on these strains, aCRY was found to act as a negative regulator in mating ability and in maintenance of mating ability, in contrast to the blue light photoreceptor phototropin that acts positively in these processes. This work also showed that mainly blue, and to a lesser degree red light are involved in zygote germination, during which the zygote undergoes meiosis. In addition to phototropin, both aCRY and plant cryptochrome were found to positively regulate zygote germination. These results clearly suggest that not only phototropin but also aCRY and plant cryptochrome are key regulators of the sexual cycle of C. reinhardtii.