Termination of transcription of the coliphage T7 "early" operon in vitro: slowness of enzyme release, and lack of any role for sigma

Transcription termination in Escherichia coli. Measurement of the rate of enzyme release from Rho-independent terminators

The termination/release phase of transcription must involve at least three major steps: cessation of elongation; release of the transcript; and release of the RNA polymerase. We have devised a novel method for measuring the rate of Escherichia coli RNA polymerase release during transcription termination. The method is based on a kinetic analysis of the rate of RNA synthesis during steady-state transcription. Using this method with defined transcription units, we have found that RNA polymerase release occurs rapidly from several rho-independent terminators. Enzyme release from the T7 early terminator occurs within 13(+/- 3) seconds of the cessation of elongation. Neither nusA protein nor supercoiling of the DNA template affects the rate of enzyme release. However, addition of excess sigma factor significantly increases the rate of enzyme recycling during the steady state. Since added sigma factor does not alter the rates of initiation and elongation by E. coli RNA polymerase holoenzyme, it appears that sigma factor stimulates one or more steps in the termination/release process and reduces the rate of enzyme release to a few seconds. We present evidence that suggests sigma may be directly involved in catalyzing release of the core RNA polymerase from the DNA template during transcription termination. The rapid rates of enzyme release we measure make it difficult to be certain of the exact pathway of events that occur in the termination/release phase of transcription. The most plausible pathway involves initial release of the RNA transcript followed by release of core RNA polymerase from the DNA. Studies on the properties of core polymerase-RNA complexes indicate that core polymerase and the RNA transcript probably do not dissociate as a complex from the terminator. Furthermore, these core-RNA complexes are too stable to represent significant intermediates in the termination/release pathway, at least in the early steps of the reaction.

Intermediates in transcription initiation from the E. coli lac UV5 promoter

We have used nondenaturing polyacrylamide gel electrophoresis to separate intermediates in transcription initiation that result from action of E. coli RNA polymerase on the lac UV5 promoter. The resolved gel complexes are characterized by DNAase I footprinting, protein subunit content, RNA content, and transcription ability. There are two "open" complexes, whose equilibrium ratio is a function of temperature; they differ in their ability to escape abortive cycling, but not in their DNAase I footprints. We find three "initiated" complexes, containing RNA chains at least 11 nucleotides long, and lacking the sigma subunit of RNA polymerase. These experiments provide a detailed view of the early initiation steps and their thermal regulation at the E. coli lac promoter.

Transcription termination regions of coliphage T7 DNA: the effects of nusA1

We report the effects in vivo of four segments of coliphage T7 DNA upon expression, from an upstream promoter, of galK in plasmids of the pKO family. Three of the segments carry the known major or putative distal terminators of host-dependent T7 early transcription. The fourth carries a novel terminator and maps in the late region of T7. We report the efficiencies of termination in these regions: evidence, based on studies with the E. coli nusA1 mutation, for an involvement of the transcription factor NusA in events at the major early and novel terminators: and the nature of the latter transcription signal.