Circadian Control of the Cell Cycle in the Dinoflagellate Kareniaa brevis: A Role for Blue Light and Characteristics of a Blue Light Receptor

Stephanie A. Brunelle^1,2,3 and Frances M. Van Dolah^1,2

¹ Marine Biomedicine and Environmental Sciences Center, Medical University of South Carolina, Charleston, SC

² Marine Biotoxins Program, NOAA, Center for Coastal Environmental Health and Biomolecular Research, Charleston,      ³ Graduate Program in Marine Biology, College of Charleston, Charleston, SC

The molecular mechanisms controlling the cell cycle in the Florida red tide dinoflagellate, Karenia brevis, are of interest because they ultimately regulate the rate of formation of toxic algal blooms.  Previous work in our laboratory has shown that the cell cycle in K. brevis is phased to the diel cycle, such that cells enter the cell cycle at precise times relative to the onset of light.  Here, we demonstrate that the cell cycle is under control of a circadian rhythm that is entrained by the dark/light transition.  In a number of organisms, blue light serves to entrain circadian rhythms.  Therefore, we next investigated the effect of red and blue light on cell cycle progression.  In the presence of blue light, K.brevis appears to enter S-phase early, whereas in red light, cell cycle progression is delayed in S-phase entry.  This suggests the presence of both blue and red light signaling pathways in K. brevis.  Here we characterize a blue-light receptor identified through EST (expressed sequence tag) screening of a K. brevis cDNA library. Cryptochromes are blue-light receptors found in bacteria, plants and animals.  The K. brevis ESTs have highest homology to a newly identified class of cryptochrome called Cry DASH.  Phylogenetic analysis of the photolyase/blue light receptor gene family shows that the K .brevis cryptochrome falls within the cryptochrome DASH clade and not the photolyase, cryptochrome 1 or cryptochrome 2 clades.  Members of the Cry DASH class are generally localized to the mitochondria or chloroplast and have DNA binding activity suggestive of a transcriptional regulatory activity.  Other classes of cryptochromes have been shown to be under circadian control, with rhythmic oscillations in gene expression.  K. brevis Cry DASH did not display either diel or circadian changes in transcription as assessed using quantitative Real-Time PCR.  This is the first blue light receptor to be identified in a dinoflagellate.  Extensive high throughput sequencing (25,000 sequences) of two K. brevis cDNA libraries, one prepared from cells harvested during the dark-phase and one from the light-phase of the diel cycle, has failed to identify a sequence with homology to known red light receptors.