REGULATION OF GROWTH PROCESSES DURING THE CELL CYCLE OF THE CHLOROCOCCAL ALGA SCENEDESMUS QUADRICAUDA UNDER A DNA REPLICATION BLOCK

Abstract

ABSTRACT The kinetics of two growth parameters (total RNA and total protein accumulation) was followed in synchronized cultures of the chlorococcal alga Scenedesmus quadricauda (Turp.) Bréb. under conditions of inhibited DNA replication in the presence of 5‐fluorodeoxyuridine (25 mg.L‐1). In the control culture, growth processes occurred in several steps with a decreasing rate of accumulation of RNA and protein amount approximately at each doubled value of the preceding step. Oscillations in the rate of growth processes in the control culture were temporally related to the initiation of individual reproductive steps. At each doubling, the cell became committed to triggering a sequence of reproductive processes, starting with DNA replication and ending with protoplast fission. Three commitment points were attained in the control culture and, consequently, three replication rounds of DNA followed by three nuclear divisions and three protoplast fissions occurred during one cell cycle. If 5‐fluorodeoxyuridine (FdUrd) was added at the beginning of the cell cycle, no reproductive processes occurred, and the cells remained uninuclear with one genome and did not divide. RNA accumulation did not seem to be affected by the presence of FdUrd for at least one cell cycle, and three or four doublings in the amount of RNA occurred during this period. Protein accumulation was even more independent of reproductive processes in the cell and continued for a period of about two or three cell cycles, attaining six doublings at the end of this period. Therefore, oscillations in the rate of protein or RNA accumulation remained even if reproductive processes were inhibited.