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Widespread backtracking by RNA pol II is a major effector of gene activation, 5’ pause release, termination and transcription elongation rate

ABSTRACT: In addition to phosphodiester bond formation, RNA polymerase II has an RNA endonuclease activity, stimulated by TFIIS, which rescues complexes that have arrested and backtracked. We identified backtracking sites using mNET-seq in human cells expressing dominant-negative TFIIS (TFIISDN) that inhibits RNA cleavage and stabilizes backtracked complexes. Backtracking is most frequent within 2 kb of start sites, consistent with slow elongation early in transcription, and in 3’ flanking regions where termination is enhanced by TFIISDN. Inhibition of the rescue from backtracked pol II by TFIISDN impaired the escape from 5’ pause sites, the completion of long transcripts, and activation of stress-inducible genes. TFIISDN slowed elongation rates genome-wide by half as measured by sequencing nascent transcripts pulse labeled with Bromouridine (Bru-seq) and by anti-pol II ChIP-seq at time points after release of a DRB block. These results suggest that rescue of backtracked pol II by TFIIS is a major stimulus of elongation.

ORGANISM(S): Homo sapiens

PROVIDER: GSE120201 | GEO | 2018/11/30

REPOSITORIES: GEO

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Project description:We developed native elongating transcript sequencing (NET-seq, Churchman and Weissman Nature 2011, PMID: 21248844) combined with RNase footprinting of nascent transcripts (RNET-seq) to visualize translocation dynamics and nascent transcript errors in paused RNA polymerases in E. coli. We employed RNET-seq to the wild-type (WT) E. coli strain and to an isogenic strain deficient in genes for GreA and GreB (ΔgreAB). Gre factors and their eukaryotic analog TFIIS rescue backtracked complexes of RNAP. Briefly, the cells were rapidly lysed via spheroplasting, and the transcribing RNAPs were released from the genomic DNA by digestion with DNase I. Any ribosomes involved in co-transcriptional translation were separated from RNAP by digestion with RNase A. All RNAPs including those associated with the fragmented double-stranded DNAs and their 5’-truncated nascent RNAs were immobilized on Ni2+-NTA beads through the hexa-histidine-tagged β’ subunit and then extensively washed with a high-salt buffer. The 5’ ends of the transcripts in ECs were trimmed with RNase T1/V1 to leave a minimal length of RNA protected by RNAP. The RNases were subsequently removed by further washing of the beads. Elution with imidazole generated ECs carrying ~6-30 nt long transcripts. The nascent RNAs isolated from the ΔgreAB strain were longer than those from the WT strain and peaked at 18 nt versus 16 nt suggesting an enrichment of backtracked ECs, which is expected to occur in the absence of Gre-dependent 3’ RNA cleavage. The RNET-seq method involves trimming of excess nascent RNA from the 5’ ends leaving only the nascent RNA that is protected by RNAP. A previous in vitro study showed that an RNAP forming an EC protects different lengths of the 3’-proximal transcript from trimming by RNases A and T1 depending on the EC translocation state. Post-translocated, pre-translocated, and backtracked complexes protect 14-nt, 15-nt and >15-nt segments of the RNA, respectively. Because the very 3’ end of the RNA is extruded to a narrow pore within front of the enzyme during backtracking, the extruded RNA remains inaccessible to RNases increasing in length as backtracking increases. Thus, paused RNAP in either the pre- and post-translocated states as well as in different backtracked distances were monitored over the entire genome. The unique properties of our RNET-seq approach provided an opportunity to dissect the core mechanisms of different types of pausing in living cells.

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Widespread backtracking by RNA pol II is a major effector of gene activation, 5’ pause release, termination and transcription elongation rate

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