S. cerevisiae adapts to different environments by translating exogenous signals into altered gene expression programs. UME3was identified in a genetic screen as a negative regulator of several early meiotic genes (SPO11, SPO13) and SSA1, a member of the HSP70 heat shock family. Also, UME3 is allelic to SSN8, a negative regulator of SUC2, and SRB11, a component of RNA polymerase II holoenzyme. These results indicate that UME3 negatively regulates diverse genes that normally respond to environmental stimuli possibly through interaction with the transcription machinery. Sequence analysis revealed that Ume3p is 38% identical to human and Drosophila "C-type" cyclins. Typical of "C-type" cyclins, UME3 mRNA and protein (Ume3p) levels do not fluctuate during the mitosis and meiosis. However, Ume3p is destroyed early in meiosis, just prior to SPO11 induction and when mitotic cultures are subjected to heat shock or carbon deprivation. This destruction of Ume3p is extremely rapid, the half life being reduced from 1.5 hours to 1 minute in cultures exposed to elevated temperatures. Deletion derivatives lacking PEST motifs exhibit a sixfold increase in stability in response to heat shock suggesting that these regions are involved in Ume3p degradation. These results suggest a model that, in response to stress or during meiosis, Ume3p is degraded to allow the expression of genes necessary for the cell to execute alternative cell fates.