Map positions of yeast genes SIR1, SIR3 and SIR4.

The HML and HMR loci in the yeast Saccharomyces cerevisiae each contain a complete copy of mating-type information. HML and HMR normally are transcriptionally inactive due to four unlinked genes, known as MAR or SIR or CMT. The map position of MAR1 (SIR2) has been reported previously; it is located on the left arm of chromosome IV, 27 cM from the centromere. Using conventional meiotic and mitotic mapping combined with recombinant DNA techniques, we have mapped three other SIR genes. SIR1 maps near the telomere of the right arm of chromosome XI; SIR3 (MAR2) maps to the right arm of chromosome XII, 31 cM distal to URA4; and SIR4 maps to the right arm of chromosome IV, 16 cM proximal to LYS4.