RESEARCH HIGHLIGHTS

Simon Bradbrook/NPG

Nature Reviews Molecular Cell Biology | AOP, published online 1 October 2014; doi:10.1038/nrm3889

GENE EXPRESSION

Histone acetylation sets Pol II H3K27ac can actively promote transcription at two temporally distinct stages

Histone post translational modifications affect gene expression; for example, histone acetylation is associated with transcriptional activation. However, whether histone modifications actively regulate RNA polymerase II (Pol II) has been difficult to ascertain, owing to the limited temporal resolution of available methods to study transcription kinetics. Having developed a fluorescent microscopy-based system that offers high temporal resolution of transcription in single living cells, Stasevich et al. now report that histone H3 Lys 27 acetylation (H3K27ac) can enhance transcription at two temporally distinct stages. In this system, which offers a temporal resolution approaching 10 seconds, mouse cells harbour a tandem gene array that is activated by a GFP-tagged glucocorticoid receptor (GFP–GR) following hormone stimulation. The recruitment of the glucocorticoid receptor to the gene array leads consecutively to Pol II recruitment and to transcription initiation and elongation. The Pol II activation stages were distinguished by different Pol II-specific fluorescent antigen-binding fragments (Fabs), and H3K27ac levels were monitored with Fab-based live endogenous modification labelling (FabLEM). The authors found high H3K27 acetylation levels at the gene array before hormone stimulation,

and these remained high for about 5 minutes after elongation began. Notably, H3K27ac levels showed great variability between cells, and cells with initially hyper-acetylated gene arrays had higher levels of GFP–GR and elongating Pol II, but not of initiating Pol II. This indicates that acetylation enhances gluco­corticoid receptor-dependent activation as well as transcription elongation, and that the two processes are mechanistically distinct because gluco­corticoid receptordependent activation did not enhance transcriptio­n initiation. The authors then quantified separately the kinetics of Pol II recruitment, initiation and elongation, revealing that Pol II ‘promoter escape’ (the transition from initiation to elongation) was almost 50% faster in gene arrays with initially high H3K27ac levels compared to those with low H3K27ac levels. Furthermore, lowering H3K27ac levels by suppressing histone methylation-mediated acetylation resulted in loss of elongating Pol II at the arrays, but not of initiating Pol II. Interestingly, the histone acetyltransferase p300 was found to localize to the gene array, and its binding partner AFF4 (AF4/FMR2 family member 4), which is part of a complex that enhances Pol II elongation, was recruited less efficiently to the gene arrays following gene activation when H3K27ac levels were low.

NATURE REVIEWS | MOLECULAR CELL BIOLOGY

Thus, AFF4 could potentially mediate the positive effect of H3K27 acetyl­ation on transcription elongation. Finally, to assess whether H3K27ac may also affect the transcription of endogenous genes, the authors performed a genome-wide analysis of gene activation by the glucocorticoid receptor, and in parallel performed chromatin-immunoprecipitation followed by sequencing (ChIP-seq) with a H3K27ac antibody. They found that the 1,000 genes most activated by the glucocorticoid receptor had hyperacetylated H3K27 before activation. Moreover, a ChIP-seq experiment with a Pol II antibody revealed the genes with hyper-acetylated H3K27 had more Pol II downstream of the promoter (elongating Pol II) than at the promoter. Together, these results demonstrate that H3K27ac can actively promote transcription at two temporally distinct stages — during activation by transcription factors and during the transition from initiation to elongation — thus potentially refining the gene activation response. It will be interesting to study how genes marked by other histone modifications, including repressive marks, respond to transcription activation. Eytan Zlotorynski ORIGINAL RESEARCH PAPER Stasevich, T. J. et al. Regulation of RNA polymerase II activation by histone acetylation in single living cells. Nature http://dx.doi.org/10.1038/nature13714 (2014)

VOLUME 15 | NOVEMBER 2014 © 2014 Macmillan Publishers Limited. All rights reserved

Gene expression: Histone acetylation sets Pol II.

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