Stress and developmental regulation of the yeast

complex (Feaver et al., 1994). In addition, a more distantly Katrina F.Cooper, Michael J.Mallory, related cyclin (Pho80p) complexes with the Pho85p Cdk Janet B.Smith1 and Randy Strich2 to repress the yeast acid phosphatase gene by inhibiting Institute for Cancer Research, Fox Chase Cancer Center, the transcriptional activator Pho4p (Hirst et al., 1994; 7701 Burholme Ave, Philadelphia, PA 19111 and 1Thomas Jefferson Kaffman et al., 1994). Finally, the yeast cyclin C–Cdk University, Department of Pathology and Cell Biology, Philadelphia, complex (Srb11p–Ume5p) is required for the repression PA 19107, USA of genes involved in sucrose utilization (Kuchin et al., 2Corresponding author 1995) and meiotic development (Strich et al., 1989; e-mail: r_strich@fccc.edu Surosky et al., 1994). This complex co-purifies with the RNA polymerase II holoenzyme (Liao et al., 1995), The ume3-1 allele was identified as a mutation that suggesting that this cyclin-Cdk represses gene expression allowed the aberrant expression of several meiotic by modifying components of the transcription machinery. genes (e.g. SPO11, SPO13) during mitotic cell division As indicated by their name, the protein and transcript in Saccharomyces cerevisiae. Here we report that UME3 levels of most cyclins that drive cell cycle progression is also required for the full repression of the HSP70 oscillate, peaking when their activity is required (for family member SSA1. UME3 encodes a non-essential review, see Nasmyth, 1993). This regulation occurs at the C-type cyclin (Ume3p) whose levels do not vary through level of both transcription and protein stability. Cyclin the mitotic cell cycle. However, Ume3p is destroyed proteolysis is mediated through ubiquitin pathways that during meiosis or when cultures are subjected to heat target at least two known motifs, PEST (proline, glutamic shock. Ume3p mutants resistant to degradation resulted acid, serine and threonine) and the destruction box. in a 2-fold reduction in SPO13 mRNA levels during Destruction boxes are required for the destruction of S meiosis, indicating that the down-regulation of this phase (A-type) and G2 (B-type) cyclins (Murray et al., cyclin is important for normal meiotic gene expression. 1989; Glotzer et al., 1991), while PEST regions (Rogers Mutational analysis identified two regions (PEST-rich et al., 1986) are carboxy-terminal domains important for and RXXL) that mediate Ume3p degradation. A third the destruction of the yeast G1 cyclins (Cln1-3p) (Tyers destruction signal lies within the highly conserved et al., 1992; Cross and Blake, 1993; Yaglom et al., 1995). cyclin box, a region that mediates cyclin–cyclin-dependPhosphorylation of the PEST regions by their associating ent kinase (Cdk) interactions. However, the Cdk activCdk appears to trigger the degradation progress (Yaglom ated by Ume3p (Ume5p) is not required for the rapid et al., 1995; Lanker et al., 1996) and, in the case of Cln3p, destruction of this cyclin. Finally, Ume3p destruction requires the yeast DnaJ homolog Ydj1p (Yaglom et al., was not affected in mutants defective for ubiquitin1996). However, the mRNA levels of the human or dependent proteolysis. These results support a model Drosophila C-type cyclins do not fluctuate during mitotic in which Ume3p, when exposed to heat shock or cell division or embryonic development (Leopold and sporulation conditions, is targeted for destruction to O’Farrell, 1991; Lew et al., 1991; Li et al., 1996). These allow the expression of genes necessary for the cell to findings may reflect their role in regulating transcription respond correctly to these environmental cues. rather than cell cycle progression.