Characterization of a Dinoflagellate Cryptochrome Blue‐light Receptor With a Possible Role in Circadian Control of the Cell Cycle¹

Karenia brevis (C. C. Davis) G. Hansen et Moestrup is a dinoflagellate responsible for red tides in the Gulf of Mexico. The signaling pathways regulating its cell cycle are of interest because they are the key to the formation of toxic blooms that cause mass marine animal die‐offs and human illness. Karenia brevis displays phased cell division, in which cells enter S phase at precise times relative to the onset of light. Here, we demonstrate that a circadian rhythm underlies this behavior and that light quality affects the rate of cell‐cycle progression: in blue light, K. brevis entered the S phase early relative to its behavior in white light of similar intensity, whereas in red light, K. brevis was not affected. A data base of 25,000 K. brevis expressed sequence tags (ESTs) revealed several sequences with similarity to cryptochrome blue‐light receptors, but none related to known red‐light receptors. We characterized the K. brevis cryptochrome (Kb CRY) and modeled its three‐dimensional protein structure . Phylogenetic analysis of the photolyase/CRY gene family showed that Kb CRY is a member of the cryptochrome DASH (CRY DASH) clade. Western blotting with an antibody designed to bind a conserved peptide within Kb CRY identified a single band at ∼55 kDa. Immunolocalization showed that Kb CRY, like CRY DASH in Arabidopsis, is localized to the chloroplast. This is the first blue‐light receptor to be characterized in a dinoflagellate. As the Kb CRY appears to be the only blue‐light receptor expressed, it is a likely candidate for circadian entrainment of the cell cycle.