CLN3, not positive feedback, determines the timing of CLN2 transcription in cycling cells.

Abstract

Transcriptional activation of the budding yeast CLNl and CLN2 genes during the late GI phase of the cell cycle has been attributed to a positive feedback loop, wherein the transcription of both genes is stimulated by the accumulation of their protein products. We demonstrate that in cycling cells CLN2 does not play a role in determining the timing of its own transcriptional activation. First, we show that CLN3 alone is sufficient to maximally activate CLN2 transcription. Cells that lack functional CLNl and CLN2 genes activate the CLN2 promoter with the same kinetics and at the same size as cells in which all three CLN genes 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-dependent activation of CLN2 transcription is directed primarily through the previously identified UASl region although another cis-acting region, UAS2, also can contribute to CLN2 activation under some conditions. The ability to activate transcription of CLN2 is not a unique property of CLN3 because ectopically expressed CLN2 can both activate the endogenous CLN2 promoter and induce Start. We propose that failure of the endogenous CLN2 gene to contribute significantly to activation of its own transcription results from its relative effectiveness at inducing Start, cell cycle progression and, subsequently, inactivation of CLN2 expression.

Acetyl-CoA induces transcription of the key G1 cyclin CLN3 to promote entry into the cell division cycle in Saccharomyces cerevisiae

“…1A). These data are consistent with previous reports showing that CLN3 is expressed rapidly in response to fresh nutrients and subsequently activates the other G1 cyclins, CLN1 and CLN2 (40)(41)(42). We therefore hypothesized that CLN3 transcription might be similarly driven by the acetyl-CoA-induced acetylation of histones present in its regulatory region.…”

A novel Mcm1-dependent element in the SWI4, CLN3, CDC6, and CDC47 promoters activates M/G1-specific transcription.

“…Mcml also controls expression of two genes, SWI4 and CLN3, which are known to be critical for entry into S phase and the transcription of the G1 cyclins CLN1 and CLN2 (Dirick et al 1995;Stuart and Wittenberg 1996). Our studies with CLN3 and SWI4 heterozygotes indicate that these proteins are not in excess during the cell cycle.…”

“…Cln3 and Swi4 play critical roles in activating the burst of CLN1 CLN2 transcription in late G1 (Nasmyth and Dirick 1991;Ogas et al 1991;Dirick et al 1995;Stuart and Wittenberg 1996), but the mechanism of that activation is not understood. We have shown that SWI4 and CLN3 transcription is ECB regulated and peaks just before the peak of CLN1 and CLN2 transcription.…”

“…Cln3 stands out among the cyclins because even small changes in its expression have the effect of either speeding or delaying the transition from G1 to S phase (Nash et al 1988;Cross 1988Cross , 1989. The only known activity of the Cln3 kinase is in activating the burst of G1/S-specific cyclin transcription that is mediated by the Swi4/Swi6 and Mbpl/Swi6 transcription complexes (Dirick et al 1995;Stuart and Wittenberg 1996), but neither the kinase substrate nor its mechanism of activation in late G1 has been identified.…”

Molecular basis of the functional distinction between Cln1 and Cln2 cyclins

“…15,16 The levels of both proteins are controlled by the same molecular mechanisms: the CLN1 and CLN2 genes are expressed periodically during the G 1 /S transition by the transcription factor SBF, [17][18][19] and the Cln1 and Cln2 proteins are degraded via the SCF Grr1 ubiquitinligase. [20][21][22] Compared with single mutants, the cln1 cln2 double mutant shows severe growth defects, 23,24 indicating a functional overlap between these two cyclins. In fact, the extensive work performed on these cyclins has revealed that they take part in many common functions, such as induction of growth polarization and budding, [25][26][27] SPB duplication, 28 degradation of CDK-inhibitors Far1 and Sic1, [29][30][31][32] inactivation of the APC ubiquitin-ligase complex, 33,34 repression of pheromone-induced transcription or stimulation of pseudohyphal growth.…”