A dominant suppressor mutation of the met30 cell cycle defect suggests regulation of the Saccharomyces cerevisiae Met4-Cbf1 transcription complex by Met32.

Met30 is the substrate recognition subunit of the essential ubiquitin ligase SCF(Met30). The essential function of Met30 is the inactivation of the Saccharomyces cerevisiae transcription factor Met4, because fully activated Met4 induces a cell cycle arrest. Met4 regulates expression of genes involved in the sulfur assimilation pathway and coordinates the transcriptional program and cell cycle progression in response to cadmium and arsenic stress. Met4 lacks DNA binding activity and requires either Cbf1 or one of the two homologous proteins Met31 and Met32 for promoter association. Accordingly, met4 mutants, cbf1 mutants, and met31 met32 double mutants are methionine auxotroph. We isolated a truncated version of Met32 (Met32(Delta145-192)) as a dominant suppressor of the cell cycle defect of met30 mutants. Expression of Met32(Delta145-192) significantly reduced induction of Met4-regulated genes. Interestingly, both Cbf1- and Met31/32-dependent genes were affected by Met32(Delta145-192). Mechanistically, Met32(Delta145-192) prevented recruitment of Met4 to both Cbf1 and Met31/32-dependent promoters. We further demonstrated that Met32 is part of the Cbf1-Met4 complex bound to Cbf1-recruiting promoter elements and that Met31/32 are required for formation of a stable Met4-Cbf1 transcription complex. These results suggest a regulatory role of Met32 as part of the Cbf1-Met4 complex and provide molecular insight into coordination of cell cycle response and modulation of gene expression programs.