GROWTH-CONTROLLED OSCILLATION IN ACTIVITY OF HISTONE H1 KINASE DURINGTHE CELL-CYCLE OF CHLAMYDOMONAS-REINHARDTII (CHLOROPHYTA)

Synchronous cultures of the cell wall-less mutant Chlamydomonas reinha rdtii Dangeard cw 15 were grown under different mean irradiances and d ifferent illumination regimes, which produced cell cycles that differe d in the number of daughter cells released from one mother cell, in th e length of the cell cycle, and in the growth rate. During the cell cy cle, the cells reached several commitment points whose number and timi ng differed according to the particular pattern of the cell cycle. The cell volume was used as a growth parameter and increased in a stepwis e manner. Each of the steps consisted of periods of both fast and slow growth. Growth usually stopped when the cells attained a volume twice that of the preceding step. Reaching particular commitment points was coupled with the position of these points in the enlargement of cell volume. Changes in the activity of histone H1 Kinase were noted during -the cell cycles of all experimental variants, and the activities were compared with the timing of various commitment points. It was found t hat kinase activity varied markedly within a single cell cycle, attain ing maximal values when the cellular volume had doubled. Each peak in kinase activity slightly preceded the commitment to an individual sequ ence of reproductive events. In addition to the oscillations related t o cell growth, a peak of Kinase activity always occurred toward the en d of the cell cycle when multiple rounds of DNA replication mitosis, a nd cell division occurred.