Dinoflagellates are protists with permanently condensed chromosomes that lack histones and whose nuclear membrane remains intact during mitosis. These unusual nuclear characters have suggested that the typical cell cycle regulators might be slightly different than those in more typical eukaryotes. To test this, a cyclin has been isolated from the dinoflagellate Gonyaulax polyedra by functional complementation in cln123 mutant yeast. This GpCycl sequence contains two cyclin domains in its C-terminal region and a degradation box typical of mitotic cyclins. similar to other dinoflagellate genes, GpCycl has a high copy number, with similar to5000 copies found in the Gonyaulax genome. An antibody raised against the N-terminal region of the GpCYCl reacts with a 68 kDa protein on Western blots that is more abundant in cell cultures enriched for G2-phase cells than in those containing primarily G1-phase cells. indicating its cellular level follows a pattern expected for a mitotic cyclin. This is the first report of a cell cycle regulator cloned and sequenced from a dinoflagellate, and Our results suggest control of the dinoflagellate cell cycle will be very similar to that of other organisms. (C) 2004 Published by Elsevier Inc.