Nascent elongating transcript sequencing (NET-seq) show different pause registers with apyrase treatment during library preparation

Transcriptional pausing aids gene regulation by cellular RNA polymerases (RNAPs). In many bacteria, a surface-exposed domain inserted into the catalytic trigger loop (TL) of RNAP, called SI3 in Escherichia coli, modulates pausing and is essential for growth. Here we describe a viable E. coli strain lacking SI3 enabled by a suppressor TL substitution (β'Ala941→Thr; ∆SI3*). ∆SI3* increased transcription rate in vitro relative to ∆SI3, possibly explaining its viability, but retained both positive and negative effects of ∆SI3 on pausing. ∆SI3* inhibited pauses stabilized by nascent RNA structures (pause hairpins; PHs) but enhanced other pauses. Using NET-seq, we found that ∆SI3*-enhanced pauses resemble the consensus elemental pause sequence whereas sequences at ∆SI3*-suppressed pauses, which exhibited greater association with PHs, were more divergent. ∆SI3*-suppressed pauses also were associated with apparent pausing one nt upstream from the consensus sequence, often generating tandem pause sites. These '–2 pauses' were stimulated by pyrophosphate in vitro and by addition of apyrase to degrade residual NTPs during NET-seq sample processing. We propose that some pauses are readily reversible by pyrophosphorolysis or single-nucleotide cleavage. Our results document multiple ways that SI3 modulates pausing in vivo and may explain discrepancies in consensus pause sequences in some NET-seq studies. Examination of nascent transcripts in WT E. coli with apyrase treatment to remove free rNTPs after cell homogenization. A B. subtilis strain is used as spike-in control for quantitative analysis.