T7 RNA polymerase elongation complex structure and movement

Characterization of T7 RNA polymerase transcription complexes assembled on nucleic acid scaffolds

We have used synthetic oligomers of DNA and RNA to assemble nucleic acid scaffolds that, when mixed with T7 RNA polymerase, allow the formation of functional transcription complexes. Manipulation of the scaffold structure allows the contribution of each element in the scaffold to transcription activity to be independently determined. The minimal scaffold that allows efficient extension after challenge with 200 mm NaCl consists of an 8-nt RNA primer hybridized to a DNA template (T strand) that extends 5-10 nt downstream. Constructs in which the RNA-DNA hybrid is less than or greater than 8 bp are less salt-resistant, and the hybrid cannot be extended beyond 12-13 bp. Although the presence of a complementary nontemplate strand downstream of the primer does not affect salt resistance, the presence of DNA upstream decreases resistance. The addition of a 4-nt unpaired "tail" to the 5' end of the primer increases salt resistance, as does the presence of an unpaired nontemplate strand in the region that contains the 8-bp hybrid (thereby generating an artificial transcription "bubble"). Scaffold complexes having these features remain active for over 1 week in the absence of salt and exhibit many of the properties of halted elongation complexes, including resistance to salt challenge, a similar trypsin cleavage pattern, and a similar pattern of RNA-RNA polymerase cross-linking.

Major conformational changes occur during the transition from an initiation complex to an elongation complex by T7 RNA polymerase

To examine changes that occur during the transition from an initiation complex (IC) to an elongation complex (EC) in T7 RNA polymerase (RNAP), we used nucleic acid-protein cross-linking methods to probe interactions of the RNAP with RNA and DNA in a halted EC. As the RNA is displaced from the RNA-DNA hybrid approximately 9 bp upstream from the active site (at -9) it interacts with a region within the specificity loop (residues 744-750) and is directed toward a positively charged surface that surrounds residues Lys-302 and Lys-303. Surprisingly, the template and non-template strands of the DNA at the upstream edge of the hybrid (near the site where the RNA is displaced) interact with a region in the N-terminal domain of the RNAP (residues 172-191) that is far away from the specificity loop before isomerization (in the IC). To bring these two regions of the RNAP into proximity, major conformational changes must occur during the transition from an IC to an EC. The observed nucleic acid-protein interactions help to explain the behavior of a number of mutant RNAPs that are affected at various stages in the initiation process and in termination.

Structure of a T7 RNA polymerase elongation complex at 2.9 A resolution

The single-subunit bacteriophage T7 RNA polymerase carries out the transcription cycle in an identical manner to that of bacterial and eukaryotic multisubunit enzymes. Here we report the crystal structure of a T7 RNA polymerase elongation complex, which shows that incorporation of an 8-base-pair RNA-DNA hybrid into the active site of the enzyme induces a marked rearrangement of the amino-terminal domain. This rearrangement involves alternative folding of about 130 residues and a marked reorientation (about 130 degrees rotation) of a stable core subdomain, resulting in a structure that provides elements required for stable transcription elongation. A wide opening on the enzyme surface that is probably an RNA exit pathway is formed, and the RNA-DNA hybrid is completely buried in a newly formed, deep protein cavity. Binding of 10 base pairs of downstream DNA is stabilized mostly by long-distance electrostatic interactions. The structure implies plausible mechanisms for the various phases of the transcription cycle, and reveals important structural similarities with the multisubunit RNA polymerases.

Structural Basis for the Transition from Initiation to Elongation Transcription in T7 RNA Polymerase

To make messenger RNA transcripts, bacteriophage T7 RNA polymerase (T7 RNAP) undergoes a transition from an initiation phase, which only makes short RNA fragments, to a stable elongation phase. We have determined at 2.1 angstrom resolution the crystal structure of a T7 RNAP elongation complex with 30 base pairs of duplex DNA containing a "transcription bubble" interacting with a 17-nucleotide RNA transcript. The transition from an initiation to an elongation complex is accompanied by a major refolding of the amino-terminal 300 residues. This results in loss of the promoter binding site, facilitating promoter clearance, and creates a tunnel that surrounds the RNA transcript after it peels off a seven-base pair heteroduplex. Formation of the exit tunnel explains the enhanced processivity of the elongation complex. Downstream duplex DNA binds to the fingers domain, and its orientation relative to upstream DNA in the initiation complex implies an unwinding that could facilitate formation of the open promoter complex.

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.

Common structural features of nucleic acid polymerases

Structures of multisubunit RNA polymerases strongly differ from the many known structures of single subunit DNA and RNA polymerases. However, in functional complexes of these diverse enzymes, nucleic acids take a similar course through the active center. This finding allows superposition of diverse polymerases and reveals features that are functionally equivalent. The entering DNA duplex is bent by almost 90 degrees with respect to the exiting template-product duplex. At the point of bending, a dramatic twist between subsequent DNA template bases aligns the "coding" base with the binding site for the incoming nucleoside triphosphate (NTP). The NTP enters through an opening that is found in all polymerases, and, in most cases, binds between an alpha-helix and two catalytic metal ions. Subsequent phosphodiester bond formation adds a new base pair to the exiting template-product duplex, which is always bound from the minor groove side. All polymerases may undergo "induced fit" upon nucleic acid binding, but the underlying conformational changes differ.

Structural transitions mediating transcription initiation by T7 RNA polymerase

During transcription initiation, RNA polymerases appear to retain promoter interactions while transcribing short RNAs that are frequently released from the complex. Upon transition to elongation, the polymerase releases promoter and forms a stable elongation complex. Little is known about the changes in polymerase conformation or polymerase:DNA interactions that occur during this process. To characterize the transitions that occur in the T7 RNA polymerase transcription complex during initiation, we prepared enzymes with Fe-BABE conjugated at 11 different positions. Addition of H(2)O(2) to transcription complexes prepared with these enzymes led to nucleic acid strand scission near the conjugate. Changes in the cleavage sites revealed a series of conformational changes and rearrangements of protein:nucleic acid contacts that mediate progression through the initiation reaction.

Pre-existing distortions in nucleic acid structure aid polypurine tract selection by HIV-1 reverse transcriptase

Precise cleavage at the polypurine tract (PPT)/U3 junction by human immunodeficiency virus type 1 (HIV-1) reverse transcriptase RNase H is critical for generating a correct viral DNA end for subsequent integration. Using potassium permanganate (KMnO(4)) modification, we have identified a significant distortion in the nucleic acid structure at the HIV-1 PPT/U3 junction in the absence of trans-acting factors. Unusually high reactivity of template thymine +1 is detected when the PPT primer is extended by DNA or RNA at its 3' terminus. Chemical footprinting suggests that the extent of base unstacking in the wild-type species is comparable when the +1 A:T base pair is replaced by a C:T mismatch. However, reactivity of this template base is diminished after alterations to upstream (rA)(4):(dT)(4) or (rG)(6):(dC)(6) tracts. Importantly, there is a correlation between the structural deformation at base pair +1 and precise cleavage at the PPT/U3 junction by HIV-1 reverse transcriptase/RNase H. KMnO(4) modification also revealed unusually high reactivity for one of two (dT)(4):(rA)(4) duplexes upstream of the PPT/U3 junction, suggesting a significant structural distortion within the PPT itself in the absence of the retroviral polymerase. Structural abnormalities in this region are not only essential for resistance of the PPT to hydrolysis but also significantly impact the conformation of the PPT/U3 junction. Our data collectively suggest that the entire PPT sequence contributes to the structural distortion at the PPT/U3 junction, potentially providing a mechanism for its selective processing.

Translocation after synthesis of a four-nucleotide RNA commits RNA polymerase II to promoter escape

Transcription is a complex process, the regulation of which is crucial for cellular and organismic growth and development. Deciphering the molecular mechanisms that define transcription is essential to understanding the regulation of RNA synthesis. Here we describe the molecular mechanism of escape commitment, a critical step in early RNA polymerase II transcription. During escape commitment ternary transcribing complexes become stable and committed to proceeding forward through promoter escape and the remainder of the transcription reaction. We found that the point in the transcription reaction at which escape commitment occurs depends on the length of the transcript RNA (4 nucleotides [nt]) as opposed to the position of the active site of the polymerase with respect to promoter DNA elements. We found that single-stranded nucleic acids can inhibit escape commitment, and we identified oligonucleotides that are potent inhibitors of this specific step. These inhibitors bind RNA polymerase II with low nanomolar affinity and sequence specificity, and they block both promoter-dependent and promoter-independent transcription, the latter occurring in the absence of general transcription factors. We demonstrate that escape commitment involves translocation of the RNA polymerase II active site between synthesis of the third and fourth phosphodiester bonds. We propose that a conformational change in ternary transcription complexes occurs during translocation after synthesis of a 4-nt RNA to render complexes escape committed.

Promoter clearance by T7 RNA polymerase. Initial bubble collapse and transcript dissociation monitored by base analog fluorescence

Footprinting, fluorescence, and x-ray structural information from the initial, promoter-bound complex of T7 RNA polymerase describes the very beginning of the initiation of transcription, whereas recent fluorescence and biochemical studies paint a preliminary picture of an elongation complex. The current work focuses on the transition from an initially transcribing, promoter-bound complex to an elongation complex clear of the promoter. Fluorescence quenching is used to follow the melted state of the DNA bubble, and a novel approach using a locally mismatched fluorescent base analog reports on the local structure of the heteroduplex. Fluorescent base analogs placed at positions -2 and -1 of the promoter indicate that this initially melted, nontranscribed region remains melted as the polymerase translocates through to position +8. In progressing to position +9, this region of the DNA bubble begins to collapse. Probes placed at positions +1 and +2 of the template strand indicate that the 5' end of the RNA remains in a heteroduplex as the complex translocates to position +10. Subsequent translocation leads to sequential dissociation of the first 2 bases of the RNA. These results show that the initially transcribing complex bubble can reach a size of up to 13 base pairs and a maximal heteroduplex length of 10 base pairs. They further indicate that initial bubble collapse precedes dissociation of the 5' end of the RNA.

T7 promoter release mediated by DNA scrunching

Transcription initiation includes a phase in which short transcripts dissociate from the transcription complex and the polymerase appears not to move away from the promoter. During this process DNA may scrunch within the complex or the polymerase may transiently break promoter contacts to transcribe downstream DNA. Promoter release allowing extended downstream movement of the polymerase may be caused by RNA-mediated disruption of promoter contacts, or by limits on the amount of DNA that can be scrunched. Using exonuclease and KMnO4 footprinting of T7RNAP transcription complexes we show that the DNA scrunches during progression through initial transcription. To determine whether promoter release is determined by RNA length or by the amount of DNA scrunched, we compared release at promoters where the polymerase is forced to initiate at +2 with those where it initiates at +1. For RNAs of identical length, release is greater when more DNA is scrunched. Release is inhibited when a nick introduced into the template relieves the strain of scrunching. DNA scrunching therefore makes an important contribution to T7 promoter release.

Structure in nascent RNA leads to termination of slippage transcription by T7 RNA polymerase

T7 RNA polymerase presents a very simple model system for the study of fundamental aspects of transcription. Some time ago it was observed that in the presence of only GTP as a substrate, on a template encoding the initial sequence GGGA., T7 RNA polymerase will synthesize a 'ladder' of poly-G RNA products. At each step, the ratio of elongation to product release is consistently approximately 0.75 until the RNA reaches a length of approximately 13-14 nt, at which point this ratio drops precipitously. One model to explain this drop in complex stability suggests that the nascent RNA may be structurally hindered by the protein; the RNA may be exiting via a pathway not taken by normally synthesized RNA and therefore becomes sterically destabilized. The fact that the length of RNA at which this occurs is close to the length at which the transition to a stably elongating complex occurs might have led to other mechanistic proposals. Here we show instead that elongation falls off due to the cooperative formation of structure in the nascent RNA, most likely an intramolecular G-quartet structure. Replacement of GTP by 7-deaza-GTP completely abolishes this transition and G-ladder synthesis continues with a constant efficiency of elongation beyond the limit of detection. The polymerase-DNA complex creates no barrier to the growth of the nascent (slippage) RNA, rather termination is similar to that which occurs in rho-independent termination.

Fluorescence characterization of the transcription bubble in elongation complexes of T7 RNA polymerase

The various kinetic and thermodynamic models for transcription elongation all require an understanding of the nature of the melted bubble which moves with the RNA polymerase active site. Is the general nature of the bubble system-dependent or are there common energetic requirements which constrain a bubble in any RNA polymerases? T7 RNA polymerase is one of the simplest RNA polymerases and is the system for which we have the highest-resolution structural information. However, there is no high-resolution information available for a stable elongation complex. In order to directly map melted regions of the DNA in a functionally paused elongation complex, we have introduced fluorescent probes site-specifically into the DNA. Like 2-aminopurine, which substitutes for adenine bases, the fluorescence intensity of the new probe, pyrrolo-dC, which substitutes for cytosine bases, is sensitive to its environment. Specifically, the fluorescence is quenched in duplex DNA relative to its fluorescence in single-stranded DNA, such that the probe provides direct information on local melting of the DNA. Placement of this new probe at specific positions in the non-template strand shows clearly that the elongation bubble extends about eight bases upstream of the pause site, while 2-aminopurine probes show that the elongation bubble extends only about one nucleotide downstream of the last base incorporated. The positioning of the active site very close to the downstream edge of the bubble is consistent with previous studies and with similar studies of the promoter-bound, pre-initiation complex. The results show clearly that the RNA:DNA hybrid can be no more than eight nucleotides in length, and characterization of different paused species suggests preliminarily that these dimensions are not sequence or position dependent. Finally, the results confirm that the ternary complex is not stable with short lengths of transcript, but persists for a substantial time when paused in the middle or at the (runoff) end of duplex DNA.

The T7 RNA polymerase intercalating hairpin is important for promoter opening during initiation but not for RNA displacement or transcription bubble stability during elongation

The recently described crystal structures of a T7RNAP-promoter complex and an initial transcription complex reveal a beta-hairpin which inserts between the template and nontemplate strands of the promoter [Cheetham, G. M., et al. (1999) Nature 399, 80; Cheetham, G. M., et al. (1999) Science 286, 2305]. A stacking interaction between the exposed DNA bases and a valine at the tip of this hairpin may be especially important for stabilizing the opened promoter during initiation. It has been suggested that this hairpin may also be important for holding the transcription bubble open during transcript elongation, and a proposed model for how the RNA exits the transcription complex implies that this hairpin may also help displace the RNA from the template strand. To test these hypotheses, we have characterized both point and deletion mutants of this element. We find that these mutants exhibit reduced activity on linear, double-stranded templates but not on supercoiled or partially single-stranded templates. Probing of promoter-polymerase complexes, initial transcription complexes, and elongation complexes with KMnO(4) and a single-strand specific endonuclease reveals that the mutants have greatly reduced promoter unwinding activity during initiation. However, the structure and stability of the transcription bubble during elongation are not altered in the mutant enzymes, and RNA displacement activity is also normal. Thus, the T7RNAP intercalating hairpin is important, though not essential, for stabilizing the opened promoter during initiation, but is not important for RNA displacement or for transcription bubble structure or stability during elongation.

Sequence-specific termination by T7 RNA polymerase requires formation of paused conformation prior to the point of RNA release

BACKGROUND

The sequence-specific, hairpin-independent termination signal for the bacteriophage RNA polymerases in Escherichia coli rrnB t1 terminator consists of two modules. The upstream module includes the conserved sequence and the downstream one is U-rich.

RESULTS

Elongation complexes of T7 RNA polymerase paused 2 bp before reaching the termination site at a 500 microM concentration of NTP. At 5-50 microM NTP, however, they paused and terminated there or resumed elongation beyond the termination site. Only at higher concentrations of NTP (500 microM), the pause complex proceeded slowly to and became incompetent at the termination site. At 4 bp or more before the termination site, the unprotected single-stranded region of transcription bubble shrank at the trailing edge to 4-5 bp from approximately 10 bp, resulting from duplex formation of the conserved sequence. The pause and bubble collapse were not observed with an inactive mutant of the termination signal.

CONCLUSION

Sequence-specific termination requires the slow elongation mode of paused conformation, working only at high concentrations of NTP for a few bp prior to the RNA release site. The collapse of bubble that was observed several base pairs before the termination site and/or the resulting duplex might subsequently lead to the paused conformation of T7 elongation complexes.

Scanning mutagenesis reveals roles for helix n of the bacteriophage T7 RNA polymerase thumb subdomain in transcription complex stability, pausing, and termination

Deletions within the thumb subdomain (residues 335-408) of T7 RNA polymerase decrease elongation complex stability and processivity, but the structure of a T7RNAP initial transcription complex containing a 3-nucleotide RNA reveals no interactions between the thumb and the RNA or DNA. Modeling of a longer RNA in this structure, using a T7DNAP-primer-template structure as a guide, suggests that the phosphate ribose backbone of the RNA contacts a stretch of mostly positively charged side chains between residues 385 and 395 of helix N of the thumb. Scanning mutagenesis of this region reveals that alanine substitutions of Arg(391), Ser(393), and Arg(394) destabilize elongation complexes and that substitutions at 393 and 394 increase termination of transcripts 5 or more bases in length. The alpha-carbons of all 3 of these residues lie on the side of helix N, which faces into the template-binding cleft of the RNA polymerase, and modeling suggests that they can contact the RNA 4-5 bases away from the 3'-end. Alanine substitutions of other residues within 385-395 do not have marked effects on transcription complex stability, but alanine substitutions of Asp(388) and Tyr(385) reduce pausing and termination at the T7 concatemer junction. Both of these side chains lie on the outer side of helix N, pointing away from the template binding cleft. The thumb subdomain of T7RNAP therefore has roles both in transcription complex stabilization and in pausing and termination at the T7 concatemer junction.