GENE REGULATION Sequence, Chromatin, Action!

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GENE REGULATION Sequence, chromatin, action!

When a cell receives a signal that explaining how these genes can be corepressor at these genes prevents alters its transcriptional programme, induced without SWI–SNF remod- productive transcription in the primary response genes (PRGs) elling. These PRGs are generally absence of TLR4 signalling, and that are rapidly induced. Two recent inducible by a wide range of stimuli, it is the acetylation of histones at spe- papers provide new insights into whereas those that require remodel- cific residues in response to induc- the basis of the speedy activation of ling are more specifically activated. tion that provides the rapid switch to mammalian PRGs. Ramirez-Carrozzi et al. propose that corepressor removal and production Although some mammalian independence from SWI–SNF com- of mature transcripts. Consistent PRGs depend on chromatin remod- plexes is key to the more promiscu- with the Ramirez-Carrozzi et al. elling by SWI–SNF complexes for ous activation of CpG-containing study, these rapidly responding inducible expression, others do not. promoters. PRGs tend to have high CG levels in By studying PRGs that are induced In a second study, Hargreaves and their promoters. by Toll-like receptor 4 (TLR4) colleagues focused on pre-association These studies make important signalling in mouse macrophages, of RNA polymerase II (RNAPII) steps towards understanding how Ramirez-Carrozzi et al. determined with promoters, a phenomenon that sequence and chromatin features that dependency on SWI–SNF has been proposed to poise genes of genes relate to the dynamics of remodelling is correlated with the for expression. Also looking at TLR4 gene expression. Future studies that absence of a CpG island in the pro- signalling, the authors identified a set look at responses to a wider range of moter. Even in unstimulated cells, of rapidly responding PRGs at which stimuli and at genes with different the promoters of TLR4-responsive RNAPII binding and active histone expression dynamics will add further genes that contain a CpG island had marks are seen in the absence of detail to this picture. features of constitutively active chro- induction. However, contrary to what Louisa Flintoft matin; furthermore, CpG islands has been observed for Drosophila

were assembled into chromatin melanogaster genes, RNAPII is not ORIGINAL RESEARCH PAPERS with reduced nucleosome stability paused at the initiation stage. Instead, Ramirez-Carrozzi, V. R. et al. A unifying model for in vitro. The authors propose that full-length unspliced transcripts are the selective regulation of inducible transcription by CpG islands and nucleosome the presence of a CpG island, in produced, and regulation occurs at remodeling. Cell 138, 114–128 (2009) | combination with binding of con- the level of mRNA elongation and Hargreaves, D. C. et al. Control of inducible gene expression by signal-dependent transcriptional stitutively expressed transcription processing. The authors provide elongation. Cell 138, 129–145 (2009) factors, results in active chromatin, evidence that the presence of a