Transcriptional Regulation of Metabolic Genes by the Basic Leucine Zipper Transcription Factor Hac1ip and Nutrient Stimuli

Saccharomyces cerevisiae cells respond to nutrients in their environment by altering their metabolic and transcriptional state in order to optimise the use of available nutrients and decide which of the several developmental pathways to pursue. In the yeast S. cerevisiae, meiosis and pseudohyphal growth are two major differentiation outcomes in response to nitrogen starvation. A central component of unfolded protein response pathway, the bZIP transcription factor Hac1ip, negatively regulates meiosis and pseudohyphal growth. The present study investigates this negative regulatory mechanism at early meiotic genes by Hac1ip in nitrogen-rich conditions. Regulation of transcription by Ume6p transcriptional regulator, Rpd3p-Sin3p histone deacetylase complex and Isw2p-Itc1p chromatin remodelling complex at URS1 was also investigated here. We also tested for induction of pseudohyphal growth in diploids from SK1 genetic background in response to nitrogen starvation conditions known to induce meiosis. I constructed destabilised β-galactosidase reporters expressed from URS1- CYC1-Ub-X-lacZ reporters to analyze transcriptional activity at URS1 site of early meiotic genes in nutrient rich conditions. The data presented here successfully demonstrates Hac1ip-mediated repression at URS1 sites in nitrogen-rich conditions. URS1-CYC1-Ub-X-lacZ reporters were expressed in mitotic repression machinery mutants (ume6Δ, rpd3Δ, sin3Δ, isw2Δ and itc1Δ) under nitrogen rich conditions. The data presented here from these experiments not only corroborates their known role in repression at URS1 but also suggested regulation at additional sites in the minimal CYC1 promoter. Deletion of Sin3p suggested independent repression function separable from Rpd3p. Isw2p also acts independently of Itc1p at sites other than URS1. We also show that pseudohyphal growth was stimulated by non-fermentable carbon sources in sporulation efficient SK1 genetic background. The data also indicates that stimulation of pseudohyphal growth by non-fermentable carbon sources does not require respiration function or functional mitochondrial RTG pathway