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The Snf1 protein kinase of Saccharomyces cerevisiae has been shown to have a role in regulating haploid invasive growth in response to glucose depletion. Cells contain three forms of the Snf1 kinase, each with a different beta-subunit isoform, either Gal83, Sip1, or Sip2. We present evidence that different Snf1 kinases play distinct roles in two aspects of invasive growth, namely, adherence to the agar substrate and filamentation. The Snf1-Gal83 form of the kinase is required for adherence, whereas either Snf1-Gal83 or Snf1-Sip2 is sufficient for filamentation. Genetic evidence indicates that Snf1-Gal83 affects adherence by antagonizing Nrg1- and Nrg2-mediated repression of the FLO11 flocculin and adhesin gene. In contrast, the mechanism(s) by which Snf1-Gal83 and Snf1-Sip2 affect filamentation is independent of FLO11. Thus, the Snf1 kinase regulates invasive growth by at least two distinct mechanisms.

The Snf1 protein kinase of Saccharomyces cerevisiae is important for many cellular responses to glucose limitation, including haploid invasive growth. We show here that Snf1 regulates transcription of FLO11, which encodes a cell surface glycoprotein required for invasive growth. We further show that Nrg1 and Nrg2, two repressor proteins that interact with Snf1, function as negative regulators of invasive growth and as repressors of FLO11. We also examined the role of Snf1, Nrg1, and Nrg2 in two other Flo11-dependent processes. Mutations affected the initiation of biofilm formation, which is glucose sensitive, but also affected diploid pseudohyphal differentiation, thereby unexpectedly implicating Snf1 in a response to nitrogen limitation. Deletion of the NRG1 and NRG2 genes suppressed the defects of a snf1 mutant in all of these processes. These findings suggest a model in which the Snf1 kinase positively regulates Flo11-dependent developmental events by antagonizing Nrg-mediated repression of the FLO11 gene.

Sip4 is a Zn(2)Cys(6) transcriptional activator that binds to the carbon source-responsive elements of gluconeogenic genes in Saccharomyces cerevisiae. The Snf1 protein kinase interacts with Sip4 and regulates its phosphorylation and activator function in response to glucose limitation; however, evidence suggested that another kinase also regulates Sip4. Here we examine the role of the Srb10 kinase, a component of the RNA polymerase II holoenzyme that has been primarily implicated in transcriptional repression but also positively regulates Gal4. We show that Srb10 is required for phosphorylation of Sip4 during growth in nonfermentable carbon sources and that the catalytic activity of Srb10 stimulates the ability of LexA-Sip4 to activate transcription of a reporter. Srb10 and Sip4 coimmunoprecipitate from cell extracts and interact in two-hybrid assays, suggesting that Srb10 regulates Sip4 directly. We also present evidence that the Srb10 and Snf1 kinases interact with different regions of Sip4. These findings support the view that the Srb10 kinase not only plays negative roles in transcriptional control but also has broad positive roles during growth in carbon sources other than glucose.