Histone H2B Ubiquitylation Regulates Histone Gene Expression by Suppressing Antisense Transcription in Fission Yeast

Histone H2B monoubiquitylation (H2Bub1) is tightly linked to RNA polymerase II transcription elongation, and is also directly implicated in DNA replication and repair. Loss of H2Bub1 is associated with defects in cell cycle progression, but how these are related to its various functions, and the underlying mechanisms involved, is not understood. Here we describe a role for H2Bub1 in the regulation of replication-dependent histone genes in the fission yeast Schizosaccharomyces pombe. H2Bub1 activates histone genes indirectly by suppressing antisense transcription of ams2(+)-a gene encoding a GATA-type transcription factor that activates histone genes and is required for assembly of centromeric chromatin. Mutants lacking the ubiquitylation site in H2B or the H2B-specific E3 ubiquitin ligase Brl2 had elevated levels of ams2(+) antisense transcripts and reduced Ams2 protein levels. These defects were reversed upon inhibition of Cdk9-an ortholog of the kinase component of positive transcription elongation factor b (P-TEFb)-indicating that they likely resulted from aberrant transcription elongation. Reduced Cdk9 activity also partially rescued chromosome segregation phenotypes of H2Bub1 mutants. In a genome-wide analysis, loss of H2Bub1 led to increased antisense transcripts at over 500 protein-coding genes in H2Bub1 mutants; for a subset of these, including several genes involved in chromosome segregation and chromatin assembly, antisense derepression was Cdk9-dependent. Our results highlight antisense suppression as a key feature of cell cycle-dependent gene regulation by H2Bub1, and suggest that aberrant transcription elongation may underlie the effects of H2Bub1 loss on cell cycle progression.