Evidence for Continuous Availability of the Phosphoribosylglycinamide Synthetase Structural Gene for Transcription during the Cell Cycle of the Eucaryote Chlorella

In synchronous cells of Chlorella pyrenoidosa (strain 7-11-05), phosphoribosylglycinamide synthetase activity increased in a continuous, linear manner with a change in rate of linear accumulation occurring at the onset of the period of DNA replication during the cell cycle. The increase in enzyme activity was dependent upon protein and RNA synthesis. Phosphoribosylglycinamide synthetase activity remained constant during a 6-hour period in which protein and RNA synthesis were inhibited in separate experiments. These data indicate that the enzyme is stable in vivo under cell cycle conditions. Thus, the linear pattern of accumulation of this stable enzyme (in vivo) and its change in rate of linear accumulation at the onset of DNA replication provide evidence that the structural gene of this enzyme is continuously available for transcription (and is under nearly a constant state of repression) during the cell cycle, and that this gene is early replicating and is transcribed shortly after its replication.