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The repression of transcription of the silent mating-type locus HMRa in the yeast Saccharomyces cerevisiae requires the four SIR proteins, histone H4 and a flanking site designated HMR-E. The SUM1-1 mutation alleviated the need for many of these components in transcriptional repression. In the absence of each of the SIR proteins, SUM1-1 restored repression in MAT alpha strains; thus, SUM1-1 appeared to bypass the need for the SIR genes in repression of HMRa. Repression was not specific to the genes normally present at HMR, since the TRP1 gene placed at HMR was repressed by SUM1-1 in a sir3 strain. Therefore, like the mechanisms of silencing normally used at HMR, silencing by SUM1-1 was gene-nonspecific. SUM1-1 suppressed point mutations in histone H4, but failed to suppress strongly a deletion mutation in histone H4. Similarly, SUM1-1 suppressed mutations in the three known elements of HMR-E, but was unable to suppress a deletion of HMR-E. These epistasis analyses implied that the functions required for repression at HMR can be ordered, with the SIR genes and silencer elements acting upstream of SUM1-1. SUM1-1 itself may function at the level of chromatin in the assembly of inactive DNA at the silent mating-type loci.

SUM1-1: a suppressor of silencing defects in Saccharomyces cerevisiae.