Both transcription factors and histones can be phosphorylated during inducible gene expression in response to extracellular stimuli, leading to transcription factor activation and chromatin remodeling. Josefowicz et al. found that specific phosphorylation of a transcription factor and of a histone are both required for regulation of gene expression in response to a pathogen signal. Mass spectrometry analysis of histones from mouse macrophages stimulated with bacterial lipopolysaccharide (LPS) identified increased phosphorylation of histone H3 at serine 28 (H3S28), without detecting phosphorylation status changes in other residues of histone H3 or other histones. Chromatin immunoprecipitation with DNA sequencing (ChIP-seq) and RNA sequencing revealed that H3S28 was phosphorylated at the transcription start sites of rapidly transcribed genes involved in inflammation and regulation of cytokine production but not at constitutively expressed housekeeping genes. H3S28 phosphorylation depended on mitogen- and stress-associated protein kinases 1 and 2 (MSK1/2) but not ribosomal S6 kinases, as shown by siRNA knockdown of these proteins and pharmacological inhibition of MSKs. MSKs also phosphorylate transcription factors involved in inflammation, including p65, a subunit of nuclear factor κB (NF-κB). In an in vitro chromatin-based assay of histone acetyltransferase p300-dependent transcription, MSK1-dependent phosphorylation of either p65 or histone H3 increased transcription, with additive effects when both were phosphorylated, and transcription decreased when a mutant H3 lacking the relevant phosphorylation site was used. Histone acetylation at specific sites, such as those modified by p300, is associated with enhanced gene expression. The presence of phosphorylated H3S28 correlated with acetylation of H3K27, a proxy for p300 histone acetyltransferase activity, suggesting that H3S28 phosphorylation may promote transcription by recruiting p300. Pharmacological inhibition of MSK impaired the LPS-mediated induction of gene expression in macrophages, as well as reduced the secretion of several inflammatory factors. Thus, the same kinase can promote gene expression by targeting both transcription factors and chromatin in response to the same signal. Future studies will determine whether this has been an overlooked aspect in the control of gene expression.
S. Z. Josefowicz, M. Shimada, A. Armache, C. H. Li, R. M. Miller, S. Lin, A. Yang, B. D. Dill, H. Molina, H.-S. Park, B. A. Garcia, J. Taunton, R. G. Roeder, C. D. Allis, Chromatin kinases act on transcription factors and histone tails in regulation of inducible transcription. Mol. Cell 64, 347–361 (2016). [PubMed]