Abstract
The yeast Saccharomyces cerevisiae grows at widely varying rates in different growth media. In order to maintain a relatively constant cell size, yeast cells must regulate the rate of progress through the cell cycle to match changes in growth rate, moving quickly through G1 in rich medium, and slowly in poor medium. We have examined connections between nutrients, and the expression and activity of Cln3-Cdc28 kinase that regulates the G1-S boundary of the cell cycle in yeast, a point referred to as Start. We find that Cln3 protein levels are highest in glucose and lower in poorer carbon sources. This regulation involves both transcriptional and post-transcriptional control. Although the Ras-cAMP pathway does not appear to affect CLN3 transcription, cAMP increases Cln3 protein levels and Cln3-Cdc28 kinase activity. This regulation requires untranslated regions of the CLN3 message, and can be explained by changes in protein synthesis rates caused by cAMP. A model for CLN3 regulation and function is presented in which CLN3 regulates G1 length in response to nutrients.