Specific threonine-4 phosphorylation and function of RNA polymerase II CTD during M phase progression.

Dynamic phosphorylation of Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 heptad-repeats in the C-terminal domain (CTD) of the large subunit coordinates progression of RNA polymerase (Pol) II through the transcription cycle. Here, we describe an M phase-specific form of Pol II phosphorylated at Thr4, but not at Tyr1, Ser2, Ser5, and Ser7 residues. Thr4 phosphorylated Pol II binds to centrosomes and midbody and interacts with the Thr4-specific Polo-like kinase 1. Binding of Pol II to centrosomes does not require the CTD but may involve subunits of the non-canonical R2TP-Prefoldin-like complex, which bind to and co-localize with Pol II at centrosomes. CTD Thr4 mutants, but not Ser2 and Ser5 mutants, display severe mitosis and cytokinesis defects characterized by multipolar spindles and polyploid cells. We conclude that proper M phase progression of cells requires binding of Pol II to centrosomes to facilitate regulation of mitosis and cytokinesis in a CTD Thr4-P dependent manner.

Tyrosine phosphorylation of RNA polymerase II CTD is associated with antisense promoter transcription and active enhancers in mammalian cells.

In mammals, the carboxy-terminal domain (CTD) of RNA polymerase (Pol) II consists of 52 conserved heptapeptide repeats containing the consensus sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Post-translational modifications of the CTD coordinate the transcription cycle and various steps of mRNA maturation. Here we describe Tyr1 phosphorylation (Tyr1P) as a hallmark of promoter (5' associated) Pol II in mammalian cells, in contrast to what was described in yeast. Tyr1P is predominantly found in antisense orientation at promoters but is also specifically enriched at active enhancers. Mutation of Tyr1 to phenylalanine (Y1F) prevents the formation of the hyper-phosphorylated Pol IIO form, induces degradation of Pol II to the truncated Pol IIB form, and results in a lethal phenotype. Our results suggest that Tyr1P has evolved specialized and essential functions in higher eukaryotes associated with antisense promoter and enhancer transcription, and Pol II stability.DOI: http://dx.doi.org/10.7554/eLife.02105.001.

Cyclin-dependent kinase 9 links RNA polymerase II transcription to processing of ribosomal RNA.

Ribosome biogenesis is a process required for cellular growth and proliferation. Processing of ribosomal RNA (rRNA) is highly sensitive to flavopiridol, a specific inhibitor of cyclin-dependent kinase 9 (Cdk9). Cdk9 has been characterized as the catalytic subunit of the positive transcription elongation factor b (P-TEFb) of RNA polymerase II (RNAPII). Here we studied the connection between RNAPII transcription and rRNA processing. We show that inhibition of RNAPII activity by α-amanitin specifically blocks processing of rRNA. The block is characterized by accumulation of 3' extended unprocessed 47 S rRNAs and the entire inhibition of other 47 S rRNA-specific processing steps. The transcription rate of rRNA is moderately reduced after inhibition of Cdk9, suggesting that defective 3' processing of rRNA negatively feeds back on RNAPI transcription. Knockdown of Cdk9 caused a strong reduction of the levels of RNAPII-transcribed U8 small nucleolar RNA, which is essential for 3' rRNA processing in mammalian cells. Our data demonstrate a pivotal role of Cdk9 activity for coupling of RNAPII transcription with small nucleolar RNA production and rRNA processing.

Threonine-4 of mammalian RNA polymerase II CTD is targeted by Polo-like kinase 3 and required for transcriptional elongation.

Eukaryotic RNA polymerase II (Pol II) has evolved an array of heptad repeats with the consensus sequence Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7 at the carboxy-terminal domain (CTD) of the large subunit (Rpb1). Differential phosphorylation of Ser2, Ser5, and Ser7 in the 5' and 3' regions of genes coordinates the binding of transcription and RNA processing factors to the initiating and elongating polymerase complexes. Here, we report phosphorylation of Thr4 by Polo-like kinase 3 in mammalian cells. ChIPseq analyses indicate an increase of Thr4-P levels in the 3' region of genes occurring subsequently to an increase of Ser2-P levels. A Thr4/Ala mutant of Pol II displays a lethal phenotype. This mutant reveals a global defect in RNA elongation, while initiation is largely unaffected. Since Thr4 replacement mutants are viable in yeast we conclude that this amino acid has evolved an essential function(s) in the CTD of Pol II for gene transcription in mammalian cells.

Polycomb associates genome-wide with a specific RNA polymerase II variant, and regulates metabolic genes in ESCs.

Polycomb repressor complexes (PRCs) are important chromatin modifiers fundamentally implicated in pluripotency and cancer. Polycomb silencing in embryonic stem cells (ESCs) can be accompanied by active chromatin and primed RNA polymerase II (RNAPII), but the relationship between PRCs and RNAPII remains unclear genome-wide. We mapped PRC repression markers and four RNAPII states in ESCs using ChIP-seq, and found that PRC targets exhibit a range of RNAPII variants. First, developmental PRC targets are bound by unproductive RNAPII (S5p(+)S7p(-)S2p(-)) genome-wide. Sequential ChIP, Ring1B depletion, and genome-wide correlations show that PRCs and RNAPII-S5p physically bind to the same chromatin and functionally synergize. Second, we identify a cohort of genes marked by PRC and elongating RNAPII (S5p(+)S7p(+)S2p(+)); they produce mRNA and protein, and their expression increases upon PRC1 knockdown. We show that this group of PRC targets switches between active and PRC-repressed states within the ESC population, and that many have roles in metabolism.

Interferon antagonist NSs of La Crosse virus triggers a DNA damage response-like degradation of transcribing RNA polymerase II.

La Crosse encephalitis virus (LACV) is a mosquito-borne member of the negative-strand RNA virus family Bunyaviridae. We have previously shown that the virulence factor NSs of LACV is an efficient inhibitor of the antiviral type I interferon system. A recombinant virus unable to express NSs (rLACVdelNSs) strongly induced interferon transcription, whereas the corresponding wt virus (rLACV) suppressed it. Here, we show that interferon induction by rLACVdelNSs mainly occurs through the signaling pathway leading from the pattern recognition receptor RIG-I to the transcription factor IRF-3. NSs expressed by rLACV, however, acts downstream of IRF-3 by specifically blocking RNA polymerase II-dependent transcription. Further investigations revealed that NSs induces proteasomal degradation of the mammalian RNA polymerase II subunit RPB1. NSs thereby selectively targets RPB1 molecules of elongating RNA polymerase II complexes, the so-called IIo form. This phenotype has similarities to the cellular DNA damage response, and NSs was indeed found to transactivate the DNA damage response gene pak6. Moreover, NSs expressed by rLACV boosted serine 139 phosphorylation of histone H2A.X, one of the earliest cellular reactions to damaged DNA. However, other DNA damage response markers such as up-regulation and serine 15 phosphorylation of p53 or serine 1524 phosphorylation of BRCA1 were not triggered by LACV infection. Collectively, our data indicate that the strong suppression of interferon induction by LACV NSs is based on a shutdown of RNA polymerase II transcription and that NSs achieves this by exploiting parts of the cellular DNA damage response pathway to degrade IIo-borne RPB1 subunits.

Transcribing RNA polymerase II is phosphorylated at CTD residue serine-7.

RNA polymerase II is distinguished by its large carboxyl-terminal repeat domain (CTD), composed of repeats of the consensus heptapeptide Tyr1-Ser2-Pro3-Thr4-Ser5-Pro6-Ser7. Differential phosphorylation of serine-2 and serine-5 at the 5' and 3' regions of genes appears to coordinate the localization of transcription and RNA processing factors to the elongating polymerase complex. Using monoclonal antibodies, we reveal serine-7 phosphorylation on transcribed genes. This position does not appear to be phosphorylated in CTDs of less than 20 consensus repeats. The position of repeats where serine-7 is substituted influenced the appearance of distinct phosphorylated forms, suggesting functional differences between CTD regions. Our results indicate that restriction of serine-7 epitopes to the Linker-proximal region limits CTD phosphorylation patterns and is a requirement for optimal gene expression.