ACTIVATION OF CLN1 AND CLN2 G(1) CYCLIN GENE-EXPRESSION BY BCK2

The Saccharomyces cerevisiae CLN3 protein, a G(1) cyclin, positively regulates the expression of CLN1 and CLN2, two additional G(1) cyclins whose expression during late G(1) is activated, in part, by the transcription factors SW14 and SW16. We isolated 12 complementation groups of mutants that require CLN3. The members of one of those complementation groups hale 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 cu-factor, and hale 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 G(1) with very low levels of CLN1 and CLN2 RNA. The slow growth rate and long G(1) 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 G(1).