Abstract
Mating-type switching (MTS) in fission yeast Schizosaccharomyces pombe is a highly regulated gene conversion event. In the process, heterochromatic donors of genetic information are selected based on the P or M cell type and on the use of two recombination enhancers, SRE2 promoting use of mat2-P and SRE3 promoting use of mat3-M. Recently, we found that the histone H3K4 methyltransferase complex Set1C participates in donor selection, raising the question of how a complex best known for its effects in euchromatin controls recombination in heterochromatin. Here, we report that the histone H2BK119 ubiquitin ligase complex HULC functions with Set1C in MTS, as mutants in the shf1, brl1, brl2 and rad6 genes showed defects similar to Set1C mutants and belonged to the same epistasis group as set1Δ. Moreover, using H3K4R and H2BK119R histone mutants and a Set1-Y897A catalytic mutant indicated that ubiquitylation of histone H2BK119 by HULC and methylation of histone H3K4 by Set1C are functionally coupled in MTS. Cell-type biases in mutants further showed that the regulation might be by inhibiting use of the SRE3 enhancer in M cells, in favor of SRE2.
Consistently, imbalanced switching in the mutants was traced to compromised association of the directionality factor Swi6 with the recombination enhancers in M cells. Based on their known effects at other chromosomal locations, we speculate that HULC and Set1C might control nucleosome mobility and strand invasion near the SRE elements. In addition, we uncovered distinct effects of HULC and Set1C on histone H3K9 methylation and gene silencing, consistent with additional functions in the heterochromatic domain.
Author Summary Mating-type switching in the fission yeast Schizosaccharomyces pombe occurs by gene conversion using a donor, mat2 or mat3 located in a heterochromatin region. Multiple studies have shown that donor selection is critically affected by heterochromatic factors. Here, we document the role of euchromatic factors, the histone H2BK119 ubiquitin ligase complex HULC and the H3K4 methyltransferase complex Set1C, in donor selection. Mutational analysis indicated that HULC and Set1C inhibit donor choice at the mat3 cis-acting recombination enhancer SRE3 in M cells, through concerted histone modifications by the two complexes. Linking this effect with heterochromatin, mutants in each complex, shf1Δ and set1Δ strains, exhibited decreased association of the HP1 heterochromatic factor Swi6 with the SRE2 and SRE3 recombination enhancers at the mat2 and mat3 silent loci in M cells. These joint effects by the two complexes on MTS were observed even though other aspects, the methylation state of histone H3K9 and effects on gene silencing differed between the shf1Δ and set1Δ strains. The results provide insight into the regulation of recombination by chromatin structure.