A DETAILED STUDY OF THE COMPLEX CELL-CYCLE OF THE DINOFLAGELLATE CRYPTHECODINIUM-COHNII BIECHELER AND EVIDENCE FOR VARIATION IN HISTONE-H1 KINASE-ACTIVITY

By adding the protein synthesis inhibitor, emetine (10(-4) M) to a hig hly synchronized population of Crypthecodinium cohnii Biecheler 1938 a t different phases of its cycle, we were able to determine: 1. The exi stence and the lengthening of the G2-Phase (30 min) in the first cycle (cycle with swimming G1 phase). 2. The time of the second cell cycle phases (cycle in the cyst): G1, 30 min; S, 1.5 h; G2, 2 h and M, 2 h. These results, together with the estimation of the cell volume of the two and four swimming daughter cells emerging from the cysts, allowed us to state the existence of two transition points: G1/S and G2/M, whi ch are necessary for completion of mitosis. We completed this refined approach of the cell cycle in studying the activities of the histone H 1 kinase either in dividing or in non-dividing Crypthecodinium cohnii cells with either total soluble proteins or the isolated mitotic kinas e complex. The H1 kinase activity of this purified complex is noticeab ly higher (twice as high) in the dividing cells than in the non-dividi ng ones. These data are discussed in the light of the basic characteri stics of the dinokaryon, and also compared with recent biochemical obs ervations on the same organism and studies on other higher eukaryotic protists and metazoa.