ISOLATION AND PARTIAL CHARACTERIZATION OF CONDITIONAL CELL-DIVISION CYCLE MUTANTS IN CHLAMYDOMONAS

We have isolated a number of temperature conditional cell division cycle mutants of the unicellular plant Chlamydomonas reinhardtii that are defective in single nuclear genes. Cells grow and divide normally at the permissive temperature (21 degrees C), but arrest in division at the restrictive temperature (33 degrees C). We have characterized these mutants using DNA probes and immunofluorescence techniques to localize cytoskeletal and microtubule organizing centre proteins. We describe here 3 broad classes of cell cycle mutation which result in cell cycle arrest with: unreplicated DNA (G1 arrest), duplicated DNA(G2 arrest) and multiple nuclei due to defective cytokinesis (cytokinesis arrest). The continuation of nuclear division in mutants blocked in cytokinesis provides support of an earlier hypothesis that stage specific events in the Chlamydomonas cell cycle are arranged in separate dependent sequences. The mutants isolated in the present study provide insights into the role of cytoskeletal proteins in the coordination of plant cell division and the means to investigate the molecular mechanisms whereby division by multiple fission is controlled in the unicellular plant Chlamydomonas.