Fig. 4a. The upper diagram, which is NOT to scale, shows the mating type loci on S. cerevisiae chromosome III. HML is near the left end of the chromosome, MAT is near the middle of the chromosome, and HMR is near the right end of the chromosome. E and I refer to the cis-acting loci that set up heterochromatic chromatin structures at HML and HMR. RE refers to the Recombination Enhancer that, when turned on in a cells, stimulates recombination in the left half of chromosome III, thus favoring mating type switching with HML. W, X and Z refer to homologous regions that are identical in sequence between HML, HMR and MAT. Y refers to the mating-type-specific information at each of these loci. The centromere of chromosome III is indicated in yellow. For more information, see my lecture notes and my description in class. A detailed explanation of mating type switching in S. cerevisiae is provided by Jim Haber in a recent review (Annu Rev Genet 32:561-599, 1998).

The lower diagram, which is approximately to scale (except that the sizes of the homologous regions are exaggerated) shows the mating type loci (mat1, mat2 and mat3), which are distributed over a ~35-kb region on the left arm of chromosome II. Blue arrows indicate transcribed genes. The red arrowhead indicates a polar replication fork barrier that prevents rightward-moving forks from replicating mat1. The position within the homology region to the left of mat1 where replication fork pausing and imprinting take place is indicated by black arrows. IR-L and IR-R are the inverted repeats that set the boundaries of the heterochromatic region. The CEN homology contributes substantially to establishment of the heterochromatic region. For more information, see my lecture notes and my description in class. A detailed explanation of mating type switching in S. pombe is provided by Jacob Dalgaard and Amar Klar in a recent review (Trends Genet 17:153-157, 2001).