Type II Topoisomerase activities are required in both G1 and G2/M phases of the dinoflagellate cell cycle

SUMMARY Dinoflagellates genomes are large (up to 200 pg) and are encoded in histoneless chromosomes that are quasi-permanently condensed and circular. This unique combination of chromosomal characteristics presents additional topological and cell cycle control problems for a eukaryotic cell, potentially requiring novel topoisomerase activities. The heterotrophic dinoflagellate Crypthecodinium cohnii was used in this study. The topoisomerase II activities throughout its cell cycle were investigated by DNA flow cytometry following enzyme deactivation. Fluorescence microscopy was also used for studying the chromosome morphology of treated cells. Two classes of topoisomerase II inhibitors were applied in our study, both of which caused G1 delay as well as G2/M arrest in the C. cohnii cell cycle. At high doses, the topoisomerase poisons amsacrine and ellipticine induced DNA fragmentation in C. cohnii cells. Topoisomerase II activities, as measured by the ability to decatenate kinetoplastid DNA (kDNA), are normally detected throughout the cell cycle in C. cohnii. Our results suggest that the requirement of type II topoisomerase activities during the G1 phase of the cell cycle may relate to the unwinding of quasi-permanently condensed chromosomes for the purpose of transcription.