R-loops are marks for histone H3 Ser10 phosphorylation and chromatin compaction

R-loops are transcription by-products that may constitute a threat to genome integrity. In addition to specific enzymes to remove them, eukaryotes rely on a number of mRNP biogenesis factors such as the THO complex, to prevent co-transcriptional R-loop formation. We show in Saccharomyces cerevisiae that R-loops are tightly and specifically linked with histone H3-Ser10 phosphorylation (H3S10P), a mark of chromatin condensation. Importantly, ChIP-chip analyses reveal a clear H3S10P accumulation at the pericentromeric chromatin during the G1-phase of the cell cycle only in R loop-accumulating yeast strains but not in those non-accumulating R-loops, and a significantly higher accumulation during S-phase. Such a difference can also be detected in a number of genes along the genome. ChIP-chip studies were perfomed with antibodies against Histone H3 and the phosphorylated Histone H3 at Serine10 in the yeast S. cerevisiae.