Mechanisms of Very Long Abortive Transcript Release during Promoter Escape.

A phage T5 N25 promoter variant, DG203, undergoes the escape transition at the +16 to +19 positions after transcription initiation. By specifically examining the abortive activity of the initial transcribing complex at position +19 (ITC19), we observe the production of both GreB-sensitive and GreB-resistant VLAT19. This suggests that ITC19, which is perched on the brink of escape, is highly unstable and can achieve stabilization through either backtracking or forward translocation. Of the forward-tracked fraction, only a small percentage escapes normally (followed by stepwise elongation) to produce full-length RNA; the rest presumably hypertranslocates to release GreB-resistant VLATs. VLAT formation is dependent not only on consensus -35/-10 promoters with 17 bp spacing but also on sequence characteristics of the spacer DNA. Analysis of DG203 promoter variants containing different spacer sequences reveals that AT-rich spacers intrinsically elevate the level of VLAT formation. The AT-rich spacer of DG203 joined to the -10 box presents an UP element sequence capable of interacting with the polymerase α subunit C-terminal domain (αCTD) during the escape transition, which in turn enhances VLAT release. Utilization of the spacer/-10 region UP element by αCTD subunits requires a 10-15 bp hypertranslocation. We document the physical occurrence of hyper forward translocation using ExoIII footprinting analysis.

Sequence-Dependent Promoter Escape Efficiency Is Strongly Influenced by Bias for the Pretranslocated State during Initial Transcription.

Abortive transcription initiation can be rate-limiting for promoter escape and therefore represents a barrier to productive gene expression. The mechanism for abortive initiation is unknown, but the amount of abortive transcript is known to vary with the composition of the initial transcribed sequence (ITS). Here, we used a thermodynamic model of translocation combined with experimental validation to investigate the relationship between ITS and promoter escape on a set of phage T5 N25 promoters. We found a strong, negative correlation between RNAP's propensity to occupy the pretranslocated state during initial transcription and the efficiency of promoter escape (r = -0.67; p < 10(-6)). This correlation was almost entirely caused by free energy changes due to variation in the RNA 3' dinucleotide sequence at each step, implying that this sequence element controls the disposition of initial transcribing complexes. We tested our model experimentally by constructing a set of novel N25-ITS promoter variants; quantitative transcription analysis again showed a strong correlation (r = -0.81; p < 10(-6)). Our results support a model in which sequence-directed bias for the pretranslocated state during scrunching results in increased backtracking, which limits the efficiency of promoter escape. This provides an answer to the long-standing issue of how sequence composition of the ITS affects promoter escape efficiency.

Monitoring abortive initiation.

Abortive initiation, when first discovered, was an enigmatic phenomenon, but fully three decades hence, it has been shown to be an integral step in the transcript initiation process intimately tied to the promoter escape reaction undergone by RNA polymerase at the initiation-elongation transition. A detailed understanding of abortive initiation-promoter escape has brought within reach a full description of the transcription initiation mechanism. This enormous progress was the result of convergent biochemical, genetic, and biophysical investigations propelled by parallel advances in quantitation technology. This chapter discusses the knowledge gained through the biochemical approach and a high resolution method that yields quantitative and qualitative information regarding abortive initiation-promoter escape at a promoter.

Promoter Escape by Escherichia coli RNA Polymerase.

Promoter escape is the process that an initiated RNA polymerase (RNAP) molecule undergoes to achieve the initiation-elongation transition. Having made this transition, an RNAP molecule would be relinquished from its promoter hold to perform productive (full-length) transcription. Prior to the transition, this process is accompanied by abortive RNA formation-the amount and pattern of which is controlled by the promoter sequence information. Qualitative and quantitative analysis of abortive/productive transcription from several Escherichia coli promoters and their sequence variants led to the understanding that a strong (RNAP-binding) promoter is more likely to be rate limited (during transcription initiation) at the escape step and produce abortive transcripts. Of the two subelements in a promoter, the PRR (the core Promoter Recognition Region) was found to set the initiation frequency and the rate-limiting step, while the ITS (the Initial Transcribed Sequence region) modulated the ratio of abortive versus productive transcription. The highly abortive behavior of E. coli RNAP could be ameliorated by the presence of Gre (transcript cleavage stimulatory) factor(s), linking the first step in abortive RNA formation by the initial transcribing complexes (ITC) to RNAP backtracking. The discovery that translocation during the initiation stage occurs via DNA scrunching provided the source of energy that converts each ITC into a highly unstable "stressed intermediate." Mapping all of the biochemical information onto an X-ray crystallographic structural model of an open complex gave rise to a plausible mechanism of transcription initiation. The chapter concludes with contemplations of the kinetics and thermodynamics of abortive initiation-promoter escape.

An alternate mechanism of abortive release marked by the formation of very long abortive transcripts.

The Esigma70-dependent N25 promoter is rate-limited at promoter escape. Here, RNA polymerase repeatedly initiates and aborts transcription, giving rise to a ladder of short RNAs 2-11 nucleotides long. Certain mutations in the initial transcribed sequence (ITS) of N25 lengthen the abortive initiation program, resulting in the release of very long abortive transcripts (VLATs) 16-19 nucleotides long. This phenomenon is completely dependent on sequences within the first 20 bases of the ITS since altering sequences downstream of +20 has no effect on their formation. VLAT formation also requires strong interactions between RNA polymerase and the promoter. Mutations that change the -35 and -10 hexamers and the intervening 17 base pair spacer away from consensus decrease the probability of aborting at positions +16 to +19. An unusual characteristic of the VLATs is their undiminished levels in the presence of GreB, which rescues abortive RNAs (

Initial transcribed sequence mutations specifically affect promoter escape properties.

Promoter escape efficiency of E. coli RNA polymerase is guided by both the core promoter and the initial transcribed sequence (ITS). Here, we quantitatively examined the escape properties of 43 random initial sequence variants of the phage T5 N25 promoter. The position for promoter escape on all N25-ITS variants occurred at the +15/+16 juncture, unlike the +11/+12 juncture for the wild type N25. These variants further exhibited a 25-fold difference in escape efficiency. ITS changes favoring promoter escape showed a compositional bias that is unrelated to nucleotide substrate binding affinity for the initial positions. Comparing all variants, the natural N25 promoter emerges as having evolved an ITS optimal for promoter escape, giving a high level of productive synthesis after undergoing the shortest abortive program. We supplemented GreB to transcription reactions to better understand abortive initiation and promoter escape in vivo. GreB supplementation elevated productive RNA synthesis 2-5-fold by altering the abortive RNA pattern, decreasing the abundance of the medium (6-10 nt) to long (11-15 nt) abortive RNAs without changing the levels of short (2-5 nt) and very long abortive RNAs (16-20 nt). The GreB-refractive nature of short abortive RNA production may reflect a minimum length requirement of 4-5 bp of the RNA-DNA hybrid for maintaining the stability of initial or backtracked complexes. That the very long abortive RNAs are unaffected by GreB suggests that they are unlikely to be products of polymerase backtracking. How the ITS might influence the course of early transcription is discussed within the structural context of an initial transcribing complex.

In vitro studies of transcript initiation by Escherichia coli RNA polymerase. 1. RNA chain initiation, abortive initiation, and promoter escape at three bacteriophage promoters.

RNA chain initiation and promoter escape is the latter stage of transcription initiation. This stage is characterized by several well-defined biochemical events: synthesis and release of short RNA products ranging 2 to 15 nucleotides in length, release of the sigma subunit from the enzyme-promoter complex, and initial translocation of the polymerase away from the promoter. In this paper, we report the use of a steady-state transcription assay with [gamma-(32)P]ATP labeling to subject the RNA chain initiation-promoter escape reaction to quantitative analysis. The specific parameters we follow to describe the chain initiation-promoter escape process include the abortive and productive rates, the abortive probability, the abortive:productive ratio, and the maximal size of the abortive product. In this study, we measure these parameters for three bacteriophage promoters transcribed by Escherichia coli RNA polymerase: T7 A1, T5 N25, and T5 N25(antiDSR). Our studies show that all three promoters form substantial amounts of abortive products under all conditions we tested. However, each of the promoters shows distinct differences from the others when the various parameters are compared. At 100 microM NTP, in a 10 min reaction, the abortive and productive yields are 87 and 13%, respectively, for T7 A1; 97 and 3%, respectively, for T5 N25; and 99.4 and 0.6%, respectively, for T5 N25(antiDSR). These values correspond to approximately 7, 32, and 165 abortive transcripts per productive transcript for the three promoters, respectively. The yield of most of the abortive products is not affected by the elevated concentration of the NTP substrate corresponding to the next template-specified nucleotide; hence, abortive products are not normally formed through a simple process of "kinetic competition". Instead, formation of abortive products appears to be determined by intrinsic DNA signals embedded in the promoter recognition region and the initial transcribed sequence region of each promoter.

In vitro studies of transcript initiation by Escherichia coli RNA polymerase. 2. Formation and characterization of two distinct classes of initial transcribing complexes.

By following the kinetics of abortive and productive synthesis in single-round transcription assays, we confirm the existence of two general classes of initial transcribing complexes (ITCs), which we term "productive ITC" and "unproductive ITC". The productive ITCs are able to escape from the promoter rapidly to produce full-length transcripts, but only after carrying out an obligate series of abortive initiation steps. The unproductive ITCs were found to synthesize mostly abortive transcripts of 2-3 nucleotides and escape from the promoter extremely slowly, if at all. Formation of the unproductive ITC is not due to the inactive RNA polymerase. Instead, RNA polymerase molecules recovered from both the productive and unproductive ITC fractions were shown to carry out abortive and productive synthesis with both the partitioning tendency and transcription kinetics similar to those of the original enzyme. Our results suggest that early transcription complexes are partitioned into the productive and unproductive ITCs most likely during the formation of open promoter complexes. The extent of partitioning varies with individual promoter sequences and is dependent on the nature and concentration of the initiating nucleotide. Thus, multiple classes of ITCs can be formed during promoter binding and transcript initiation.

In vitro studies of transcript initiation by Escherichia coli RNA polymerase. 3. Influences of individual DNA elements within the promoter recognition region on abortive initiation and promoter escape.

Abortive initiation and promoter escape are two principal biochemical reactions occurring in the latter stage of transcript initiation. We have analyzed the influences of individual DNA elements within the promoter recognition region (PRR) on these reactions by measuring the quantitative initiation parameters that describe abortive initiation and promoter escape; these parameters are the abortive rate, the productive rate, the abortive:productive ratio, the abortive probability, and the maximum size of abortive transcripts. Changes in the individual DNA elements within the PRR can have a substantial effect on each of these parameters. The discriminator region and the -10 element primarily influence the abortive probability at positions 2-5 and 6-10, respectively, while the -10 and -35 conserved hexamers and the spacer region affect the abortive probability at positions 11-15. Surprisingly, transcription of a consensus promoter invariably gives a higher abortive yield, a higher abortive probability, a longer abortive ladder, and a lower productive rate than promoter variants carrying even a single deviation in the consensus hexamers. These results suggest that strong RNA polymerase-PRR interactions stall the polymerase at the promoter, thereby reducing the rate of promoter escape and consequently enhancing the extent of abortive initiation.

Changes in conserved region 3 of Escherichia coli sigma 70 reduce abortive transcription and enhance promoter escape.

Mutations within the Escherichia coli rpoD gene encoding amino acid substitutions in conserved region 3 of the sigma(70) subunit of E. coli RNA polymerase restore normal stress responsiveness to strains devoid of the stress alarmone, guanosine-3',5'-(bis)pyrophosphate (ppGpp). The presence of a mutant protein, either sigma(70)(P504L) or sigma(70)(S506F), suppresses the physiological defects in strains devoid of ppGpp. In vitro, when reconstituted into RNA polymerase holoenzyme, these sigma mutants confer unique transcriptional properties, namely they reduce the probabilities of forming abortive RNAs. Here we investigated the behavior of these mutant enzymes during transcription of the highly abortive cellular promoter, gal P2. No differences between mutant and wild-type enzymes were observed prior to and including open complex formation. Remarkably, the mutant enzymes produced drastically reduced levels of gal P2 abortive RNAs and increased production of full-length gal P2 RNAs relative to the wild-type enzyme, leading to greatly reduced ratios of abortive to productive RNAs. These results are attributed mainly to a decreased formation of unproductive initial transcribing complexes with the mutant polymerases and increased rates of promoter escape. Altered transcription properties of these mutant polymerases arise from an alternative structure of the sigma(70) region 3.2 segment that permits efficient positioning of the nascent RNA into the RNA exit channel displacing sigma and facilitating sigma release.

Promoter clearance and escape in prokaryotes.

Promoter escape is the last stage of transcription initiation when RNA polymerase, having initiated de novo phosphodiester bond synthesis, must begin to relinquish its hold on promoter DNA and advance to downstream regions (DSRs) of the template. In vitro, this process is marked by the release of high levels of abortive transcripts at most promoters, reflecting the high instability of initial transcribing complexes (ITCs) and indicative of the existence of barriers to the escape process. The high abortive initiation level is the result of the existence of unproductive ITCs that carry out repeated initiation and abortive release without escaping the promoter. The formation of unproductive ITCs is a widespread phenomenon, but it occurs to different extent on different promoters. Quantitative analysis of promoter mutations suggests that the extent and pattern of abortive initiation and promoter escape is determined by the sequence of promoter elements, both in the promoter recognition region (PRR) and the initial transcribed sequence (ITS). A general correlation has been found that the stronger the promoter DNA-polymerase interaction, the poorer the ability of RNA polymerase to escape the promoter. In gene regulation, promoter escape can be the rate-limiting step for transcription initiation. An increasing number of regulatory proteins are known to exert their control at this step. Examples are discussed with an emphasis on the diverse mechanisms involved. At the molecular level, the X-ray crystal structures of RNA polymerase and its various transcription complexes provide the framework for understanding the functional data on abortive initiation and promoter escape. Based on structural and biochemical evidence, a mechanism for abortive initiation and promoter escape is described.