Activation of CLN1 and CLN2 G1 cyclin gene expression by BCK2

The Saccharomyces cerevisiae CLN3 protein, a G1 cyclin, positively regulates the expression of CLN1 and CLN2, two additional G1 cyclins whose expression during late G1 is activated, in part, by the transcription factors SWI4 and SWI6. We isolated 12 complementation groups of mutants that require CLN3. The members of one of these complementation groups have mutations in the BCK2 gene. In a wild-type CLN3 genetic background, bck2 mutants have a normal growth rate but have a larger cell size, are more sensitive to alpha-factor, and have a modest defect in the accumulation of CLN1 and CLN2 RNA. In the absence of CLN3, bck2 mutations cause an extremely slow growth rate: the cells accumulate in late G1 with very low levels of CLN1 and CLN2 RNA. The slow growth rate and long G1 delay of bck2 cln3 mutants are cured by heterologous expression of CLN2. Moreover, overexpression of BCK2 induces very high levels of CLN1, CLN2, and HCS26 RNAs. The results suggest that BCK2 and CLN3 provide parallel activation pathways for the expression of CLN1 and CLN2 during late G1.