Euchromatin factors HULC and Set1C affect heterochromatin organization for mating-type switching in fission yeast Schizosaccharomyces pombe

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.

Fission yeast Swi2 designates cell-type specific donor and stimulates Rad51-driven strand exchange

“…In addition, the AT-hook motifs might both foster interaction of Swi2 directly with SRE2 , and act indirectly through SRE3 to increase Swi2 occupancy in the whole region. In M cells, we suggest that Set1C also works at SRE3 to modulate chromatin conformation (Esquivel-Chavez et al ., 2022; Maki et al ., 2018). While further work is necessary to elucidate the molecular mechanisms underlying Swi2 localization, our in vitro investigations provide new clues for how Swi2 would stimulate recombination when localized at a donor locus and for its cell-type specific properties.…”

“…Recently, we reported that two euchromatin factors, Set1C and HULC, inhibit the choice of mat3-M as a donor in M cells, probably by affecting chromatin structure at SRE3 (Esquivel-Chavez et al ., 2022; Maki et al ., 2018). We analyzed the possible involvement of these euchromatin factors in the function of Swi2 (Figure 6C and Supplementary Figure S3D).…”

“…We previously showed that Set1C and HULC participate in the mat2-P donor choice by acting on SRE3 (Esquivel-Chavez et al ., 2022; Maki et al ., 2018). The current study showed that the double mutant with Swi6-binding and Set1 mutations had a slightly more severe phenotype, while the Set1 mutation slightly suppressed the P cell-bias phenotype of the AT-hook mutant (Figure 6C).…”

“…Strains were generated by transformations or genetic crosses. The swi2 mutant strains (TM366, TM511, TM519, and TM527) were generated from EM20 by the CRISPR/Cas9 method (Esquivel-Chavez et al ., 2022). EM20 was transformed with a gRNA expression plasmid (pEM59 derivative), a Cas9 expression plasmid, and donor template DNA for HR.…”

“…In addition to heterochromatin formation and the Swi2-Swi5 complex, we recently found that the euchromatic factors Set1C and HULC also affect MTS (Maki et al , 2018). Based on genetic analysis, we proposed that Set1C and HULC in M cells inhibit the mat3-M donor choice at SRE3 (Esquivel-Chavez et al , 2022).…”