Mating-type switching by homology-directed recombinational repair: a matter of choice

Abstract

In eukaryotes, all DNA transactions happen in the context of chromatin that often takes part in regulatory mechanisms. In particular, chromatin structure can regulate exchanges of DNA occurring through homologous recombination. Few systems have provided as detailed a view on this phenomenon as mating-type switching in yeast. Mating-type switching entails the choice of a template for the gene conversions of the expressed mating-type locus. In the fission yeast Schizosaccharomyces pombe, correct template choice requires two competing small recombination enhancers, SRE2 and SRE3, that function in the context of heterochromatin. These two enhancers act with the Swi2/Swi5 recombination accessory complex to initiate strand exchange in a cell-type-specific manner, from SRE2 in M cells and SRE3 in P cells. New research indicates that the Set1C complex, responsible for H3K4 methylation, and the Brl2 ubiquitin ligase, that catalyzes H2BK119 ubiquitylation, participate in the cell-type-specific selection of SRE2 or SRE3. Here, we review these findings, compare donor preference in S. pombe to the distantly related budding yeast Saccharomyces cerevisiae, and contrast the positive effects of heterochromatin on the donor selection process with other situations, where heterochromatin represses recombination.

Integrity of a heterochromatic domain ensured by its boundary elements

“…In this procedure, spores, but not vegetative cells, are stained dark by iodine vapors ( 31 ). Because heterochromatin is required for efficient mating-type switching and thus for homothallic mating and sporulation ( 22 , 32 , 33 ), the Δ K mutant epitype with established heterochromatin forms brown colonies similar to wild-type ( h 90 ) strains, while the Δ K mutant epitype lacking heterochromatin forms light mottled colonies like clr4 Δ strains ( 29 , 30 ). We found that in a Δ K mutant lacking both boundaries the dark epitype could not be readily propagated ( Fig.…”

“…Heterochromatin formation catalyzed by the histone H3K9 methyltransferase Clr4 occurs rapidly within cenH and, in a subsequent step, the heterochromatic state propagates to the rest of the domain with slower kinetics (21). Once established, heterochromatin is maintained in an RNAi-independent manner (20) to control mating-type switching and to silence the mating-type donor loci (22). The aspect of heterochromatin formation or stability that would be affected by IR-L and IR-R is not known.…”

Schizosaccharomyces pombe Assays to Study Mitotic Recombination Outcomes

“…In addition to the allele present at the mat1 locus, the mating type of a population of cells is also determined by the ability to switch and the alleles present at the silent mat2-P and mat3-M regions. Wild type h 90 cells can switch information at the mat1 locus and are mixture of M and P cells [120][121][122][123]. A population that has the M allele at the mat1 locus (mat1M) and lost the ability to switch is h -.…”

Sexual Processes in Microbial Eukaryotes

“…The fission yeast S. pombe, like the budding yeast S. cerevisiae (see above), switches mating type during vegetative growth, though they each use different mechanisms [28]. This provides S. pombe with increased mating opportunities with close relatives.…”