Myocardin-related transcription factors (MRTFs) are coactivators of serum response factor (SRF), and thereby regulate cytoskeletal gene expression in response to actin dynamics. MRTFs have also been implicated in transcription of heat shock protein (HSP)-encoding genes in fly ovaries, but the mechanisms remain unclear. Here, we demonstrate that, in mammalian cells, MRTFs are dispensable for gene induction of HSP-encoding genes. However, the widely used small-molecule inhibitors of the MRTF-SRF transcription pathway, derived from CCG-1423, also efficiently inhibit gene transcription of HSP-encoding genes in both fly and mammalian cells in the absence of MRTFs. Quantifying RNA synthesis and RNA polymerase distribution demonstrates that CCG-1423-derived compounds have a genome-wide effect on transcription. Indeed, tracking nascent transcription at nucleotide resolution reveals that CCG-1423-derived compounds reduce RNA polymerase II elongation, and severely dampen the transcriptional response to heat shock. The effects of CCG-1423-derived compounds therefore extend beyond the MRTF-SRF pathway into nascent transcription, opening novel opportunities for their use in transcription research.

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