same genes resident at the MAT locus in the middle of the chromosome. To address the mechanism of this novel position effect regulation, we have conducted a structural and genetic analysis of the SIR4 gene. We have determined the nucleotide sequence of the gene and found that it encodes a lysine-rich, serine-rich protein of 152 kilodaltons. Expression of the carboxy half of the protein complements a chromosomal nonsense mutation of sir4 but not a complete deletion of the gene. These results suggest that SIR4 protein activity resides in two portions of the molecule, but that these domains need not be covalently linked to execute their biological function. We also found that high-level expression of the carboxy domain of the protein yields dominant derepression of the silent loci. This anti-Sir activity can be reversed by increased expression of the SIR3 gene, whose product is normally also required for maintaining repression of the silent loci. These results are consistent with the hypothesis that SIR3 and SIR4 proteins physically associate to form a multicomponent complex required for repression of the silent mating-type loci.Control of mating-type loci in Saccharomyces cerevisiae presents a striking example of position effect regulation or altered gene expression as a function of chromosomal position. Genes encoding regulatory proteins that determine the mating type of the cell are present at three different locations on chromosome III (2, 29, 45) ( Fig. 1). The set of genes resident at the MAT locus are transcribed, and their products establish the cell's mating type (7,10,20,27,28,40). The same genes resident at either end of the chromosome, at loci designated HML and HMR, are not transcribed and do not contribute to the establishment or maintenance of cell type (18,27,30). These silent loci serve solely as repositories of mating-type information that normally can be activated only by transposition of a copy of the genes into the MAT locus (11,14,15,17,22,29,45).Differential expression of MAT versus HML and HMR results from repression of expression of the genes residing at the silent loci HML and HMR. The products of three genes, SIR2 (or MARI) through SIR4, unlinked to each other or to the mating-type loci, are required to maintain the silent loci in a transcriptionally inactive state (8,16,31,32). In the absence of any one of these products, both silent loci are expressed at a level equivalent to that of MAT. A fourth gene, SIR], is required for complete repression of the silent loci, but its inactivation yields only partial expression of the otherwise silent genes (13, 31).Repression of the genes resident at HML and at HMR also requires the integrity of cis-acting sites adjacent to each locus. Deletion of a site designated E, which lies to the left of each locus as they are conventionally represented, yields full expression of the genes in the adjacent locus (1, 5). Both E sites are less than 250 base pairs (bp) in length and can function in cis in an orientation-independent manner to repress expression fr...
