SPE3, WHICH ENCODES SPERMIDINE SYNTHASE, IS REQUIRED FOR FULL REPRESSION THROUGH NREDIT SACCHAROMYCES-CEREVISIAE

We previously identified a transcriptional regulatory element, which w e call NREDIT, that is required for repression of the sporulation-spec ific genes, DIT1 and DIT2, during vegetative growth of Saccharomyces c erevisiae. Repression through this element is dependent on the Ssn6-Tu p1 corepressor. In this study, we show that SIN4 contributes to NREDIT -mediated repression, suggesting that changes in chromatin structure a re, at least in part, responsible for regulation of DIT gene expressio n. In a screen for additional genes that function in repression of DIT (FRD genes), we recovered alleles of TUP1, SSN6, SIN4, and ROX3 and i dentified mutations comprising eight complementation groups of FRD gen es. Four of these FRD genes appeared to act specifically in NREDIT-med iated repression, and four appeared to be general regulators of gene e xpression. We cloned the gene complementing the frd3-1 phenotype and f ound that it was identical to SPE3 which encodes spermidine synthase. Mutant spd cells not only failed to support complete repression throug h NREDIT but also had modest defects in repression of some other genes . Addition of spermidine to the medium partially restored repression t o spe3 cells, indicating that spermidine may play a role in vivo as a modulator of gene expression. We suggest various mechanisms by which s permidine could act to repress gene expression.