GROWTH-INDEPENDENT REGULATION OF CLN3 MESSENGER-RNA LEVELS BY NUTRIENTS IN SACCHAROMYCES-CEREVISIAE

Saccharomyces cerevisiae cells regulate progress through the G(1) phas e of the cell cycle in response to nutrients, moving quickly through G (1) in rich medium and slowly in poor medium. Recent work has shown th at the levels of Cln3 protein, a G(1) cyclin, are low in cells growing in poor medium and high in cells growing rapidly in rich medium, cons istent with the previously recognized role of Cln3 in promoting passag e through Start. Cln3 protein levels appear to be regulated both trans criptionally and posttranscriptionally. We have worked to define the n utrient signals that regulate CLN3 mRNA levels. We find that CLN3 mRNA levels are high during log-phase growth in glucose medium, low in pos tdiauxic cells growing on ethanol, and sligh tly lower still in cells in stationary phase. CLN3 mRNA levels are induced by glucose in a proc ess that involves transcriptional control, requires metabolism of the glucose, and is independent of the Ras-cyclic AMP pathway. CLN3 mRNA l evels are also positively regulated by nitrogen sources, but phosphoru s and sulfur limitation do not affect CLN3 message levels.