Transcription of mRNA genes by Pol II is a tightly regulated and highly coordinated event that requires numerous factors working in conjunction to control gene-specific transcription with spatial and temporal specificity. Pol II transcription is aided by a set of general factors that function at core promoters, including TFIIA, TFIIB, TFIID, TFIIE, TFIIF, TFIIH and mediator. 1 These factors, along with promoter-specific transcriptional regulator proteins and chromatin-modifying factors, coordinate the assembly of pre-initiation complexes at the promoters of genes. More recently, a model has emerged in which Pol II transcription at many genes is also tightly regulated at steps that occur subsequent to the recruitment of Pol II and general factors to promoters, such as promoter proximal pausing.2 Central to regulating post-initiation events is phosphorylation of Ser2, Ser5 and Ser7 residues in the heptapeptide repeats of the CTD of the largest subunit of Pol II (Rpb1).3 Of the general transcription factors, TFIIH, a 10 subunit complex that contains both kinase and helicase activities, can phosphorylate Ser5 and Ser7 at promoters. 1,[4][5][6][7] The kinase activity is contained within the CAK submodule, which consists of the three subunits Cdk7, cyclin H and Mat1. The helicase subunits, ERCC2 and ERCC3, are thought to be critical for promoter melting, transcriptional initiation and promoter escape.
1Transcription is not only orchestrated by a multitude of protein factors and post-translational modifications, but also by non-coding RNAs (ncRNAs), which are an emerging class of regulators of Pol II transcription. 8 We found that B2 RNA, a ncRNA in mouse cells, functions as a transcriptional regulator Mouse B2 RNA represses RNA polymerase II (Pol II) transcription during the cellular heat shock response. B2 RNA binds Pol II, enters complexes at promoters, and keeps the polymerase from engaging the DNA. Here we show that phosphorylation of Ser5 residues in the Pol II carboxy terminal domain (CTD) decreases after heat shock at the promoter of the repressed actin gene in mouse cells, despite the continued presence of Cdk7 and cyclin H. Biochemical assays revealed that B2 RNA, when present with Pol II in promoter-bound complexes, specifically represses CTD phosphorylation by TFIIH. Transcribed by RNA polymerase III from short interspersed elements (SINEs), B2 RNA is upregulated upon heat shock and other cellular stresses.10 Also upregulated is the other predominant mouse SINE RNA, B1 RNA. B2 RNA binds directly to Pol II and potently represses transcription; by contrast, B1 RNA can bind tightly to Pol II but is not capable of transcriptional repression in vitro. 11,12 We found that in heat-shocked mouse cells, B2 RNA mediates the transcriptional repression of genes such as actin.9 Biochemical experiments to investigate the mechanism of repression showed that B2 RNA, via its interaction with Pol II, assembles into complexes at the promoters of genes and renders the complexes transcriptionally inactive.11 Within these inhibi...