CLN3, NOT POSITIVE FEEDBACK, DETERMINES THE TIMING OF CLN2 TRANSCRIPTION IN CYCLING CELLS

Transcriptional activation of the budding yeast CLN1 and CLN2 genes du ring the late G(1) phase of the cell cycle has been attributed to a po sitive feedback loop, wherein the transcription of both genes is stimu lated by the accumulation of their protein products. We demonstrate th at in cycling cells CLN2 does not play a role in determining the timin g of its own transcriptional activation. First, we show that CLN3 alon e is sufficient to maximally activate CLN2 transcription. Cells that l ack functional CLN1 and CLN2 genes activate the CLN2 promoter with the same kinetics and at the same size as cells in which all three CLN ge nes are functional. In addition, CLN2 transcription is activated with similar kinetics in cells that have CLN2 as their only functional CLN gene and in CLN-deficient cells. Promoter analysis shows that CLN3-dep endent activation of CLN2 transcription is directed primarily through the previously identified UAS1 region although another cis-acting regi on, UAS2, also can contribute to CLN2 activation under some conditions . The ability to activate transcription of CLN2 is not a unique proper ty of CLN3 because ectopically expressed CLN2 can both activate the en dogenous CLN2 promoter and induce Start. We propose that failure of th e endogenous CLN2 gene to contribute significantly to activation of it s own transcription results from its relative effectiveness at inducin g Start, cell cycle progression and, subsequently, inactivation of CLN 2 expression.