Heterochromatin at the Mating Type Locus

Heterochromatin also plays an important role in regulating mating-type switching. The fission yeast mating-type locus contains three mating-type cassettes, matl (either P or M), mat2-P and mat3-M over approximately 30 kb region on chromosome 2 (Egel 2004; Klar 1992). Depending on whether P or M information is found at matl, cells preferentially recombine either mat2-P (in a matl-M cell) or mat3-M (in a matl-P cell) with matl in a process known as switching. matl is transcriptionally active but mat2-P and mat3-M are maintained in a silent state (Fig. 2). The mating-type of a haploid cell is determined by the exchange between P and M information at the matl locus. Heterochromatin is required to maintain the 20-kb region containing mat2-P and mat3-M in a silent state as expression of both causes haploid cells to undergo an aberrant meiosis which is usually lethal when it occurs in haploid cells (Thon et al. 2005). The cenH region between mat2 and mat3 has 96% homology to dh elements at centromeres. cenH is required for efficient silencing and switching as replacement of this region by a marker gene causes variegated expression (Grewal and Klar 1997). As at centromeres, Swi6 also attracts cohesin to mat2-mat3, and mutations in cohesin subunits lead to defective mating-type switching (Nonaka et al. 2002). Furthermore, analyses suggest that heterochroma-tin influences long-range chromatin interactions between matl and the silent mat-ing-type cassettes to determine the direction of the switching event (Jia et al. 2004b).

From the above discussion it is clear that in fission yeast heterochromatin is required to form stable structures at distinct chromosomal loci in order to contribute to the normal function of these regions.

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