Genetic analysis of the shared role of CLN3 and BCK2 at the G(1)-S transition in Saccharomyces cerevisiae.

The transcription complexes SBF and MBF mediate the G(1)-S transition in the cell cycle of Saccharomyces cerevisiae. In late G(1), SBF and MBF induce a burst of transcription in a number of genes, including G(1)- and S-phase cyclins. Activation of SBF and MBF depends on the G(1) cyclin Cln3 and a largely uncharacterized protein called Bck2. We show here that the induction of SBF/MBF target genes by Bck2 depends partly, but not wholly, on SBF and MBF. Unlike Cln3, Bck2 is capable of inducing its transcriptional targets in the absence of functional Cdc28. Our results revealed promoter-specific mechanisms of regulation by Cln3, Bck2, SBF, and MBF. We isolated high-copy suppressors of the cln3 bck2 growth defect; all of these had the ability to increase CLN2 expression. One of these suppressors was the negative regulator of meiosis RME1. Rme1 induces CLN2, and we show that it has a haploid-specific role in regulating cell size and pheromone sensitivity. Genetic analysis of the cln3 bck2 defect showed that CLN1, CLN2, and other SBF/MBF target genes have an essential role in addition to the degradation of Sic1.