The yeast Saccharomyces cerevisiae grows at widely varying rates in di
fferent 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 G(1) in
rich medium, and slowly in poor medium. We have examined connections b
etween nutrients, and the expression and activity of Cln3-Cdc28 kinase
that regulates the G(1)-S boundary of the cell cycle in yeast, a poin
t referred to as Start. We find that Cln3 protein levels are highest i
n glucose and lower in poorer carbon sources. This regulation involves
both transcriptional and post-transcriptional control, Although the R
as-cAMP pathway does not appear to affect CLN3 transcription, cAMP inc
reases Cln3 protein levels and Cln3-Cdc28 kinase activity. This regula
tion requires untranslated regions of the CLN3 message, and can be exp
lained by changes in protein synthesis rates caused by cAMP, A model f
or CLN3 regulation and function is presented in which CLN3 regulates G
(1) length in response to nutrients.