Ume6 Is Required for the MATa/MATα Cellular Identity and Transcriptional Silencing in Kluyveromyces lactis

To explore the similarities and differences of regulatory circuits among budding yeasts, we characterized the role of the unscheduled meiotic gene expression 6 ( UME6 ) gene in Kluyveromyces lactis . We found that Ume6 was required for transcriptional silencing of the cryptic mating-type loci HML α and HMR a . Chromatin immunoprecipitation (ChIP) suggested that Ume6 acted directly by binding the cis -regulatory silencers of these loci. Unexpectedly, a MAT a ume6 strain was mating proficient, whereas a MAT α ume6 strain was sterile. This observation was explained by the fact that ume6 derepressed HML α 2 only weakly, but derepressed HMR a 1 strongly. Consistently, two a /α-repressed genes ( MTS1 and STE4 ) were repressed in the MAT α ume6 strain, but were expressed in the MAT a ume6 strain. Surprisingly, ume6 partially suppressed the mating defect of a MAT a sir2 strain. MTS1 and STE4 were repressed in the MAT a sir2 ume6 double-mutant strain, indicating that the suppression acted downstream of the a 1/α2-repressor. We show that both STE12 and the MAT a 2 / HMR a 2 genes were overexpressed in the MAT a sir2 ume6 strain. Consistent with the idea that this deregulation suppressed the mating defect, ectopic overexpression of Ste12 and a 2 in a MAT a sir2 strain resulted in efficient mating. In addition, Ume6 served as a block to polyploidy, since ume6 / ume6 diploids mated as pseudo a -strains. Finally, Ume6 was required for repression of three meiotic genes, independently of the Rpd3 and Sin3 corepressors.