NUTRITIONAL REGULATION OF MEIOSIS-SPECIFIC GENE-EXPRESSION IN SACCHAROMYCES-CEREVISIAE

For efficient meiosis and sporulation, yeast cells must be starved for nitrogen, and only nonfermentable carbon sources may be present. To find how these nutritional signals regulate entry into meiosis, transcript levels from meiotic inducer genes (IME1 and IME2) were examined under various culture conditions. Our results suggest (1) that IME1 RNA levels are negatively regulated by cell division or induced by cell division arrest at G1 phase, (2) that the G1 arrest is not sufficient to induce IME2 transcription, and (3) that IME1 and IME2 transcriptions are inhibited by either nitrogen or glucose. Acetate was required for full expressions of IME1 and IME2 RNAs, and this induction required de novo protein synthesis. We have cloned SME2 and SME3 by screening genes which, when present in increased dosage, bypass the nitrogen repression of sporulation. RNA blot analysis suggests that SME2 and SME3 are positive transcriptional regulators for SGA1 (a late meiotic gene encoding glucoamylase) and IME1, respectively, and that transcript levels from SME2 and SME3 were also negatively regulated by cell division in a manner similar to that found for IME1. However, diploids homozygous for sme2 or sme3 disruption mutation sporulated normally like isogenic wild-type strains, indicating that SME2 and SME3 are dispensable for meiosis.