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Proximity-dependent biotinylation map of the RNAPII CTD reveals that the primary role of Serine 2 phosphorylation is in the suppression of pervasive antisense transcription and not in RNA 3’ end processing [ChIP-seq]

ABSTRACT: The RNA polymerase II carboxy-terminal domain (CTD) consists of conserved repeats of the consensus sequence Y1-S2-P3-T4-S5-P6-S7, which can be phosphorylated to influence distinct stages of the transcription cycle, including RNA processing. Although affinity purification coupled with mass spectrometry has defined CTD-associated proteins, phospho-dependent CTD interactions have remained largely elusive. Proximity-dependent biotinylation (PDB) provides an alternative approach to identify protein-protein associations in the native cellular environment. Here we present a PDB-based map of the fission yeast RNAPII CTD in live cells. The proteomic screen identified known CTD-associated proteins, but also captured new and unexpected CTD proximal proteins. We also used PDB to identify phospho-dependent CTD interactions by using a mutant in which Ser2 was replaced by alanine in every repeat of the fission yeast CTD. Surprisingly, CTD-mediated biotinylation of most 3’ end processing factors was not affected in the S2A mutant, consistent with RNA-seq and ChIP-seq analysis indicating that CTD Ser2 phosphorylation is not required for 3’ end processing and transcription termination. Conversely, we found that CTD Ser2 phosphorylation is critical for the association between RNAPII and the histone methyltransferase Set2 during transcription elongation. We show that loss of CTD Ser2 phosphorylation disables the Set2-Clr6(II) axis, resulting in a global increase in cryptic antisense transcription that correlates with elevated levels of histone acetylation in gene bodies. Our findings reveal that the fundamental role of CTD Ser2 phosphorylation is to establish a chromatin-based repressive state that prevents cryptic intragenic transcription initiation.

ORGANISM(S): Schizosaccharomyces pombe

PROVIDER: GSE249234 | GEO | 2024/03/28

REPOSITORIES: GEO

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Project description:Transcription termination in Saccharomyces cerevisiae can be performed by at least two distinct pathways and is directed by the phosphorylation status of the carboxy-terminal domain (CTD) of RNA polymerase II (Pol II). Late termination of mRNAs is performed by the CPF/CF complex and requires CTD-Ser2 phosphorylation. Early termination of shorter cryptic unstable transcripts (CUTs) and small nucleolar RNAs (snoRNAs) is preformed by the Nrd1 complex, and requires CTD-Ser5 phosphorylation. In this study, mutants of the different termination pathways were compared by genome-wide expression analysis. Surprisingly, the expression changes observed upon loss of the CTD-Ser2 kinase Ctk1 are more similar to loss of a subunit of the Ser5P binding Nrd1-complex, than to loss of Ser2P binding factors. Tiling array analysis of ctk1Δ reveals readthrough at several hundred sites, including snoRNAs, as reported previously, but also many cryptic unstable transcripts, stable untranslated transcripts (SUTs) and other transcripts. Surprisingly, neither loss of CTK1 nor a Pol II CTD-Ser2 substitution mutant results in a global defect in termination of mRNAs, indicating that Ser2P is not essential for proper termination of most mRNAs. At snoRNA, Nrd1 location is shifted downstream in ctk1∆, indicating defective release rather than recruitment of Nrd1. Weakening the interaction between Nrd1 and Pol II rescues the readthrough in ctk1∆, likely by facilitating Nrd1 release. The termination defect is kinase activity dependent, but cannot be completely explained by loss of CTD-Ser2 phosphorylation , a major substrate of Ctk1, suggesting the involvement of an additional substrate. Mutant alleles of the elongation factor Spt5 rescue ctk1∆-dependent readthrough, indicating a role for Spt5 in this process, perhaps as a substrate of Ctk1. The results show that Ctk1 is more intimately involved in termination of small non-coding RNAs than was previously assumed and lead to a model in which Ctk1 influences Spt5 activity to achieve this.

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Proximity-dependent biotinylation map of the RNAPII CTD reveals that the primary role of Serine 2 phosphorylation is in the suppression of pervasive antisense transcription and not in RNA 3’ end processing [ChIP-seq]

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