Nutrient Regulation of Cell Growth and Cell Proliferation in Saccharomyces Cerevisiae

In yeast, nutrient conditions have a profound effect on progression through G1 phase of the cell cycle. The Sic1 Cdk inhibitor is a primary obstacle to G1, exit, and it is thought that elimination of Sic1 is mediated by the late-G1 cyclins Cln1 and Cln2, not by the upstream G1 cyclin Cln3. Despite this, I found that during growth in glucose medium, loss of Cln1 and Cln2 did not alter the timing of Sic1 destruction or G1 exit, while manipulation of Cln3 did. However, a role for Cln1 and Cln2 became much more apparent in cells growing on poorer carbon sources than glucose. These findings demonstrate a previously unrecognized link between carbon source and the mechanism by which cells exit G 1 phase of the cell cycle.