The linker histone H1 generally participates in the establishment of chromatin structure. However, of the seven somatic H1 isotypes in humans some are also implicated in the regulation of local gene expression. Histone H1 isotype 4 (H1.4) represses transcription, and its lysine residue 26 (Lys<sup>26</sup>) was found to be important in this aspect. H1.4K26 is known to be methylated and acetylated in vivo, but the enzymes responsible for these post-translational modifications and the regulatory cues that promote H1.4 residence on chromatin are poorly characterized. Here we report that the euchromatic histone lysine methyltransferase G9a/KMT1C mediates H1.4K26 mono- and dimethylation in vitro and in vivo and thereby provides a recognition surface for the chromatin-binding proteins HP1 and L3MBTL1. Moreover, we show evidence that G9a promotes H1 deposition and is required for retention of H1 on chromatin. We also identify members of the JMJD2/KDM4 subfamily of jumonji-C type histone demethylases as being responsible for the removal of H1.4K26 methylation
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