Alterations in transcription that precede and accompany encystment (E) of suspension grown A. castellanii have been investigated. Comparative studies were performed on cells undergoing spontaneous E in high density stationary phase cultures or after experimental induction of E at low cell densities by deprivation of nutrients in exponential growth. Onset of growth deceleration at high cell densities was accompanied by an increase in the cellular RNA. The maximum RNA content occurred in cells at stationary phase and subsequently declined with the appearance of cysts in the cultures. On the contrary, the RNA content in cells whose growth was immediately terminated by experimental E induction remained at a constant exponential level through 5 h postinduction and then began to decline shortly before the appearance of cysts. The mature cyst formed in stationary phase cultures and after experimental E induction contained an equivalent amount of RNA (∼50% of the exponential value). Comparison of the kinetics of [3H]uridine incorporation demonstrated that there was an abrupt reduction in the rate of uridine incorporation into RNA with onset of growth deceleration or after growth termination in experimental E induction. The reduced incorporation of uridine into RNA could not be attributed to to a reduced uptake of the isotope by the cells or an altered capacity of the cells to phosphorylate uridine. Uridine continued to be incorporated into RNA at a reduced rate in cells throughout growth deceleration, in stationary phase, and up to 12 h postexperimental induction. Considered together, these results indicate that a buildup in RNA is not necessary for induction of encystment in acanthamoeba. The accumulated RNA in stationary phase cells appears to be due to the greater reduction in the growth rate than in transcription and the absence of RNA turnover in cells during growth deceleration. Initiation of RNA turnover appears to accompany growth termination and induction of E. The results further demonstrate that the regulation of the rate of transcription is closely coordinated with the control of growth and encystment in acanthamoeba.

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