FIGURE 9-35 Arrangement of mating-type loci on chromosome III in the yeast S. cerevisiae. Silent (unexpressed) mating-type genes (either a or α) are located at the HML locus. The opposite mating-type gene is present at the silent HMR locus. When the α or a sequences are present at the MAT locus, they can be transcribed into mRNAs whose encoded proteins specify the mating-type phenotype of the cell. The silencer sequences near HML and HMR bind proteins that are critical for repression of these silent loci. Haploid cells can switch mating types in a process that transfers the DNA sequence from HML or HMR to the transcriptionally active MAT locus.