Core histone variants such as H2A.X and H3.3 serve specialized roles in chromatin processes that depend on the genomic distributions and amino acid sequence differences of the variant proteins. Modifications of the variants alters interactions with other chromatin components and thus the protein’s functions. Similarly, growing evidence suggests that amino acid sequence variance also modifies the functions of H1 linker histone family members, altering their effects on transcription, differentiation and development. These data are contrary to the notion that linker histones are redundant repressors. For example, complete loss of H1 does not result in an overall increase in transcription, implying that not all H1 variants are associated with repression. Furthermore, certain modifications of H1 variants can confer distinct roles. On the one hand, phosphorylation of H1 reportedly results in its release from chromatin and subsequent transcription of HIV-1 genes. On the other hand, recent evidence has shown that the phosphorylated H1 may in fact be associated with active promoters. This conflict suggests that different H1 isoforms may play distinct functional roles. Here, we provide the first genome- wide evidence that when phosphorylated, H1.4 remains associated with active promoters and may even play a role in transcription activation.
Using novel, highly specific antibodies, we generated the first genome-wide view of the H1.4 isoform phosphorylated at serine 187 (pS187-H1.4) in estradiol-inducible MCF7 cells. We observe that pS187-H1.4 is enriched primarily at the transcription start sites (TSSs) of genes activated by estradiol treatment, and depleted from those that are repressed. We also show that pS187-H1.4 strongly associates with ‘early estrogen response’ genes and co-localizes with active RNA polymerase signals.
Based on observations presented here, we propose that interphase phosphorylation at S187 by CDK9 represents an early event required for gene activation. This event may also be involved in release of promoter proximal polymerases to begin elongation by interacting directly with the polymerase or other parts of the transcription machinery. Although we focused on estrogen- responsive genes, taking into account previous evidence of H1.4 enrichment at promoters of pluripotency genes, we propose that H1.4 phosphorylation for gene activation may be a more general and global phenomenon
