Heat shock reduces stalled RNA polymerase II and nucleosome turnover genome-wide.

Heat shock rapidly induces expression of a subset of genes while globally repressing transcription, making it an attractive system to study alterations in the chromatin landscape that accompany changes in gene regulation. We characterized these changes in Drosophila cells by profiling classical low-salt-soluble chromatin, RNA polymerase II (Pol II), and nucleosome turnover dynamics at single-base-pair resolution. With heat shock, low-salt-soluble chromatin and stalled Pol II levels were found to decrease within gene bodies, but no overall changes were detected at transcriptional start sites. Strikingly, nucleosome turnover decreased genome-wide within gene bodies upon heat shock in a pattern similar to that observed with inhibition of Pol II elongation, especially at genes involved in the heat-shock response. Relatively high levels of nucleosome turnover were also observed throughout the bodies of genes with paused Pol II. These observations suggest that down-regulation of transcription during heat shock involves reduced nucleosome mobility and that this process has evolved to promote heat-shock gene regulation. Our ability to precisely map both nucleosomal and subnucleosomal particles directly from low-salt-soluble chromatin extracts to assay changes in the chromatin landscape provides a simple general strategy for epigenome characterization.