• Crypthecodinium cohnii;
  • dinoflagellate;
  • PCNA;
  • replication

Dinoflagellate chromosomes are permanently condensed and lack nucleosomes. These features suggest that dinoflagellate chromosomes must have an altered structural arrangement when compared to other eukaryotes and some modified DNA replication machinery to accommodate it. To investigate this possibility, proliferating cell nuclear antigen (PCNA), an essential component of the DNA replication machinery, was chosen for closer examination. A protein in the dinoflagellate Crypthecodinium cohnii Biecheler was found to react specifically with two monoclonal antibodies raised against PCNA. The observed band had a size of 55 kDa, which is far in excess of what has been described previously. Another dinoflagellate, Gymnodinium catenatum Bravo, also displayed a band of this size; however, a third species Amphidinium carterae Hulburt, had a band of lower molecular weight. The putative PCNA homolog in C. cohnii showed a nonconstitutive expression pattern. A time-course western blot using cells from a synchronized G1 population showed that protein levels peak during S phase of the cell cycle. Both C. cohnii and A. carterae displayed a strong nuclear localization as determined by immunofluorescence microscopy. The signal was present in a subpopulation of cells, supporting a cell-cycle-specific expression pattern. It is possible that the larger size of this protein in some dinoflagellates reflects the unusual cell cycle and DNA arrangement of this group.