TI - Chromosomal protein HMGN1 modulates histone H3 phosphorylation . AB - Here we demonstrate that HMGN1 , a nuclear protein that binds to nucleosomes and reduces the compaction of the chromatin fiber , modulates histone posttranslational modifications . In Hmgn1-/- cells , loss of HMGN1 elevates the steady-state levels of phospho-S10-H3 and enhances the rate of stress-induced phosphorylation of S10-H3 . In vitro , HMGN1 reduces the rate of phospho-S10-H3 by hindering the ability of kinases to modify nucleosomal , but not free , H3 . During anisomycin treatment , the phosphorylation of HMGN1 precedes that of H3 and leads to a transient weakening of the binding of HMGN1 to chromatin . We propose that the reduced binding of HMGN1 to nucleosomes , or the absence of the protein , improves access of anisomysin-induced kinases to H3 . Thus , the levels of posttranslational modifications in chromatin are modulated by nucleosome binding proteins that alter the ability of enzymatic complexes to access and modify their nucleosomal targets .