Four Genes Responsible for a Position Effect on Expression from HML and HMR in Saccharomyces cerevisiae

Mating type interconversion in Saccharomyces cerevisiae occurs by transposition of copies of the a or alpha mating type cassettes from inactive loci, HML and HMR, to an active locus, MAT. The lack of expression of the a and alpha genes at the silent loci results from repression by trans-acting regulators encoded by SIR (Silent Information Regulator) genes. In this paper we present evidence for the existence of four SIR genes. Inactivation of any of these genes leads to expression of cassettes at both HML and HMR. Unusual complementation properties are observed for a number of sir mutations. Specifically, some recessive mutations in different genes fail to complement. The correspondence between SIR1, SIR2, SIR3, SIR4 and other genes with similar roles (MAR, CMT, STE8 and STE9) is presented.     

Specific repression of the yeast silent mating locus HMR by an adjacent telomere.

The yeast silent mating loci HML and HMR are located at opposite ends of chromosome III adjacent to the telomeres. Mutations in the N terminus of histone H4 have been previously found to derepress the yeast silent mating locus HML to a much greater extent than HMR. Although differences in the a and alpha mating-type regulatory genes and in the cis-acting silencer elements do not appear to strongly influence the level of derepression at HMR, we have found that the differential between the two silent cassettes is largely due to the position of the HMR cassette relative to the telomere on chromosome III. While HML is derepressed to roughly the same extent by mutations in histone H4 regardless of its chromosomal location, HMR is affected to different extends depending upon its chromosomal positioning. We have found that HMR is more severely derepressed by histone H4 mutations when positioned far from the telomere (cdc14 locus on chromosome VI) but is only minimally affected by the same mutations when integrated immediately adjacent to another telomere (ADH4 locus on chromosome VII). These data indicate that the degree of silencing at HMR is regulated in part by its neighboring telomere over a distance of at least 23 kb and that this form of regulation is unique for HMR and not present at HML. These data also indicate that histone H4 plays an important role in regulating the silenced state at both HML and HMR.     

A position-effect control for gene transposition: state of expression of yeast mating-type genes affects their ability to switch

Mating-type switches of the yeast Saccharomyces cerevisiae occur by unidirectional transposition of copies of unexpressed mating-type genetic information, residing at HML and HMR loci, into the expressed MAT locus. The HML and HMR loci remain unchanged. In contrast, in appropriate strains where the silent loci are also allowed to express, for example in mar mutants, efficient switches of HML and HMR are shown to occur at rates equivalent to those observed for MAT. Thus the position-effect control on the direction of transposition is affected by the state of expression of the locus under study the expressed loci switch regardless of their location.     

Identification of silencer binding proteins from yeast: possible roles in SIR control and DNA replication

The 'silent' yeast mating-type loci (HML and HMR) are repressed by sequences (HMLE and HMRE) located over 1 kb from their promoters which have properties opposite those of enhancers, and are called 'silencers'. Both silencers contain autonomously replicating sequences (ARS). Silencer activity requires four trans-acting genes called SIR (silent information regulator). We have identified two DNA binding factors , SBF-B and SBF-E, which bind to known regulatory elements at HMRE. SBF-B binds to a region involved in both the silencer and ARS functions of HMRE, but doesn not bind to HMLE. This factor also binds to the unlinked ARS1 element. SBF-E recognizes a sequence found at both silencers. These results suggest that the two silencers may be composed of different combinations of regulatory elements at least one of which is common to both. Neither factor appears to be a SIR gene product. Hence the SIR proteins may not directly interact with the silencer control sites.     

Epigenetic inheritance of transcriptional states in S. cerevisiae

SIR1, one of several genes required for repression of yeast silent mating type loci, has a unique role in repression of the HML alpha locus. Single-cell assays revealed that cells with mutant alleles of SIR1, including presumptive null alleles, existed as populations of genetically identical cells whose members were in one of two different regulatory states. A minority of cells had a repressed HML alpha locus whereas the majority had a derepressed HML alpha locus. The two states were mitotically stable, although rare changes in state were observed during mitotic growth, possibly reflecting heritable changes to the HML alpha locus at or before replication. Analysis of changes in state suggests that SIR1 protein has a role in the establishment but not the maintenance of repression of silent mating type genes, whereas SIR2, SIR3, and SIR4 are required for maintenance.     

Efficient production of a ring derivative of chromosome III by the mating-type switching mechanism in Saccharomyces cerevisiae.

The mating-type switches in the yeast Saccharomyces cerevisiae occur by unidirectional transposition of replicas of unexpressed genetic information, residing at HML or HMR, into the mating-type locus (MAT). The source loci, HML and HMR, remain unchanged. Interestingly, when the HM cassettes are expressed, as in marl strains, the HML and HMR cassettes can also efficiently switch, apparently by obtaining genetic information from either of the other two cassettes (Klar et al., Cell 25:517-524, 1981). We have isolated a novel chromosome III rearrangement in heterothallic (marl ho) strains, which is also produced efficiently in marl HO cells, presumably the consequence of a recombination event between HML and HMR. The fusion results in the loss of sequences which are located distal to HML and to HMR and produces a ring derivative of chromosome III. Cells containing such a ring chromosome are viable as haploids; apparently, no essential loci are located distal to the HM loci. The fusion cassette behaves as a standard HM locus with respect to both regulation by the MAR/SIR control and its role in switching MAT.