Multiple Pathways for Suppression of Mutants Affecting G1-Specific Transcription in Saccharomyces cerevisiae

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Abstract
In the budding yeast, Saccharomyces cerevisiae, control of cell proliferation is exerted primarily during G1 phase. The G1-specific transcription of several hundred genes, many with roles in early cell cycle events, requires the transcription factors SBF and MBF, each composed of Swi6 and a DNA-binding protein, Swi4 or Mbp1, respectively. Binding of these factors to promoters is essential but insufficient for robust transcription. Timely transcriptional activation requires Cln3/CDK activity. To identify potential targets for Cln3/CDK, we identified multicopy suppressors of the temperature sensitivity of new conditional alleles of SWI6. A bck2Δ background was used to render SWI6 essential. Seven multicopy suppressors of bck2Δ swi6-ts mutants were identified. Three genes, SWI4, RME1, and CLN2, were identified previously in related screens and shown to activate G1-specific expression of genes independent of CLN3 and SWI6. The other four genes, FBA1, RPL40a/UBI1, GIN4, and PAB1, act via apparently unrelated pathways downstream of SBF and MBF. Each depends upon CLN2, but not CLN1, for its suppressing activity. Together with additional characterization these findings indicate that multiple independent pathways are sufficient for proliferation in the absence of G1-specific transcriptional activators.