Compaction of Active Chromatin Domains and Promoters by H3S10 Phosphorylation during Mitosis Preserves Interphase-Specific Chromatin Structure and Function

Mitotic chromosomes lose interphase-specific genome organization and transcription but gain histone phosphorylation, specifically H3S10p. This phosphorylation event compacts chromosomes in early mitosis by reducing inter-nucleosomal distance before the loading of condensins. However, it is unclear if H3S10p in mitosis preserves the identity of lost chromatin domains and promoters, both physically and functionally. Here, using the pre- mitotic expression of histone H3S10 and its mutants H3S10A and H3S10D, we show that H3S10p hyper-phosphorylates active promoters and spreads into super-domains A in mitosis, causing compaction of these regions. By spreading into active domains in the absence of genome organization, H3S10p retains their identity physically. Functionally, H3S10p ensures optimal closing of promoters by stabilizing the nucleosomes, thereby protecting them from excess loading of transcription machinery post-mitosis. In the H3S10p phospho-mutants, these chromatin regions fail to condense properly during mitosis. As a result, they exhibit enhanced accessibility and transcription of active genes in the next interphase. We propose that the spreading of mitotic H3S10p into active domains preserves their identity during mitosis and, in subsequent interphase, acts as a rheostat to fine-tune transcription and chromatin domain re-formation.