Constraining G1-Specific Transcription to Late G1 Phase: The MBF-Associated Corepressor Nrm1 Acts via Negative Feedback

Summary G1-specific transcription in yeast depends upon SBF and MBF. We have identified Nrm1 ( n egative r egulator of M BF targets 1), as a stable component of MBF. NRM1 ( YNR009w ), an MBF-regulated gene expressed during late G1 phase, associates with G1-specific promoters via MBF. Transcriptional repression upon exit from G1 phase requires both Nrm1 and MBF. Inactivation of Nrm1 results in prolonged expression of MBF-regulated transcripts and leads to hydroxyurea (HU) resistance and enhanced bypass of rad53 Δ- and mec1 Δ-associated lethality. Constitutive expression of a stabilized form of Nrm1 represses MBF targets and leads to HU sensitivity. The fission yeast homolog SpNrm1, encoded by the MBF target gene nrm1 + ( SPBC16A3.07c ), binds to MBF target genes and acts as a corepressor. In both yeasts, MBF represses G1-specific transcription outside of G1 phase. A negative feedback loop involving Nrm1 bound to MBF leads to transcriptional repression as cells exit G1 phase.