Conserved region 3 of Escherichia coli final sigma70 is implicated in the process of abortive transcription

Multiple-round in vitro transcription assays were performed using purified Escherichia coli RNA polymerase reconstituted with either wild-type or mutant final sigma70 proteins. These mutants, final sigma70(P504L) and final sigma70(S506F), bear single amino acid changes in conserved protein region 3. Behavior of the mutant enzymes on three test templates, bearing either the T7 A1, T5 N25, or T5 N25antiDSR promoter, were characterized. Transcription of all three promoter templates produced a pattern of specific abortive RNA species, which was qualitatively different for the mutants compared to the wild-type final sigma70 enzyme. Short abortive RNAs were produced at similar levels for mutant and wild-type enzymes. The production of longer abortive species was either reduced or increased by the mutant enzymes in a systematic manner that appears promoter-specific, and could be RNA length- or promoter distance-dependent. The process of abortive RNA transcription is thought to be tightly associated with that of promoter clearance. However, promoter clearance from these templates appears only slightly affected by the mutant enzymes. These mutants implicated region 3 of final sigma70 in the process of abortive transcription and suggest that the sequence of enzymatic events leading to the production of abortive or full-length RNA may be separable.