On the role of the Escherichia coli RNA polymerase sigma 70 region 4.2 and alpha-subunit C-terminal domains in promoter complex formation on the extended -10 galP1 promoter

Diversity of σ66-Specific Promoters Contributes to Regulation of Developmental Gene Expression in Chlamydia trachomatis

Promoter recognition by the RNA polymerase (RNAP) holoenzyme is a key step in gene regulation. In Chlamydia trachomatis, a medically important obligate intracellular bacterium, σ⁶⁶ allows the RNAP to initiate promoter-specific transcription throughout the chlamydial developmental cycle. Here, we investigated the intrinsic properties of σ⁶⁶-specific promoters with emphasis on their role in the developmental gene expression of C. trachomatis. First, we examined whether promoters that contain a 5'-T(-15)G(-14)-3' (TG) motif upstream from the -10 element appear more often than others in genes that are preferentially expressed during the early, middle, or late stages of the C. trachomatis developmental cycle. We then determined the critical genetic elements that are required for transcription initiation in vitro. We also assessed the activity of promoters in the presence of Scc4, which can directly interact with σ⁶⁶RNAP. Finally, we evaluated the promoter-specific dynamics during C. trachomatis infection using a reporter assay. These results reveal that the TG motif is an important determinant in certain early or late promoters. The TG promoters that have the -35 element are recognized by σ⁶⁶RNAP and Scc4 differently from those lacking the -35 element. Based on these properties, the σ⁶⁶-specific promoters can fall into three classes. Architectural diversity, behavioral plasticity, and the specific interplays between promoters and the σ⁶⁶RNAP likely contribute to developmental gene transcription in C. trachomatis. IMPORTANCE Meticulous promoter elucidation is required to understand the foundations of transcription initiation. However, knowledge of promoter-specific transcription remains limited in C. trachomatis. This work underscores the structural and functional plasticity of σ⁶⁶-specific promoters that are regulated by σ⁶⁶RNAP, as well as their importance in the developmental gene regulation of C. trachomatis.

Mechanisms of σ54-Dependent Transcription Initiation and Regulation

Cellular RNA polymerase is a multi-subunit macromolecular assembly responsible for gene transcription, a highly regulated process conserved from bacteria to humans. In bacteria, sigma factors are employed to mediate gene-specific expression in response to a variety of environmental conditions. The major variant σ factor, σ54, has a specific role in stress responses. Unlike σ70-dependent transcription, which often can spontaneously proceed to initiation, σ54-dependent transcription requires an additional ATPase protein for activation. As a result, structures of a number of distinct functional states during the dynamic process of transcription initiation have been captured using the σ54 system with both x-ray crystallography and cryo electron microscopy, furthering our understanding of σ54-dependent transcription initiation and DNA opening. Comparisons with σ70 and eukaryotic polymerases reveal unique and common features during transcription initiation.

RbpA relaxes promoter selectivity of M. tuberculosis RNA polymerase

The transcriptional activator RbpA associates with Mycobacterium tuberculosis RNA polymerase (MtbRNAP) during transcription initiation, and stimulates formation of the MtbRNAP-promoter open complex (RPo). Here, we explored the influence of promoter motifs on RbpA-mediated activation of MtbRNAP containing the stress-response σ^B subunit. We show that both the ‘extended −10’ promoter motif (T_-17G_-16T_-15G_-14) and RbpA stabilized RPo and allowed promoter opening at suboptimal temperatures. Furthermore, in the presence of the T_-17G_-16T_-15G_-14 motif, RbpA was dispensable for RNA synthesis initiation, while exerting a stabilization effect on RPo. On the other hand, RbpA compensated for the lack of sequence-specific interactions of domains 3 and 4 of σ^B with the extended −10 and the −35 motifs, respectively. Mutations of the positively charged residues K73, K74 and R79 in RbpA basic linker (BL) had little effect on RPo formation, but affected MtbRNAP capacity for de novo transcription initiation. We propose that RbpA stimulates transcription by strengthening the non-specific interaction of the σ subunit with promoter DNA upstream of the −10 element, and by indirectly optimizing MtbRNAP interaction with initiation substrates. Consequently, RbpA renders MtbRNAP promiscuous in promoter selection, thus compensating for the weak conservation of the −35 motif in mycobacteria.

How the avidity of polymerase binding to the -35/-10 promoter sites affects gene expression

Although the key promoter elements necessary to drive transcription in Escherichia coli have long been understood, we still cannot predict the behavior of arbitrary novel promoters, hampering our ability to characterize the myriad sequenced regulatory architectures as well as to design new synthetic circuits. This work builds upon a beautiful recent experiment by Urtecho et al. [G. Urtecho, et al, Biochemistry, 68, 1539-1551 (2019)] who measured the gene expression of over 10,000 promoters spanning all possible combinations of a small set of regulatory elements. Using these data, we demonstrate that a central claim in energy matrix models of gene expression-that each promoter element contributes independently and additively to gene expression-contradicts experimental measurements. We propose that a key missing ingredient from such models is the avidity between the -35 and -10 RNA polymerase binding sites and develop what we call a multivalent model that incorporates this effect and can successfully characterize the full suite of gene expression data. We explore several applications of this framework, namely, how multivalent binding at the -35 and -10 sites can buffer RNA polymerase (RNAP) kinetics against mutations and how promoters that bind overly tightly to RNA polymerase can inhibit gene expression. The success of our approach suggests that avidity represents a key physical principle governing the interaction of RNA polymerase to its promoter.

Controller protein of restriction-modification system Kpn2I affects transcription of its gene by acting as a transcription elongation roadblock

C-proteins control restriction-modification (R-M) systems' genes transcription to ensure sufficient levels of restriction endonuclease to allow protection from foreign DNA while avoiding its modification by excess methyltransferase. Here, we characterize transcription regulation in C-protein dependent R-M system Kpn2I. The Kpn2I restriction endonuclease gene is transcribed from a constitutive, weak promoter, which, atypically, is C-protein independent. Kpn2I C-protein (C.Kpn2I) binds upstream of the strong methyltransferase gene promoter and inhibits it, likely by preventing the interaction of the RNA polymerase sigma subunit with the -35 consensus element. Diminished transcription from the methyltransferase promoter increases transcription from overlapping divergent C-protein gene promoters. All known C-proteins affect transcription initiation from R-M genes promoters. Uniquely, the C.Kpn2I binding site is located within the coding region of its gene. C.Kpn2I acts as a roadblock stalling elongating RNA polymerase and decreasing production of full-length C.Kpn2I mRNA. Mathematical modeling shows that this unusual mode of regulation leads to the same dynamics of accumulation of R-M gene transcripts as observed in systems where C-proteins act at transcription initiation stage only. Bioinformatics analyses suggest that transcription regulation through binding of C.Kpn2I-like proteins within the coding regions of their genes may be widespread.

T7 phage factor required for managing RpoS in Escherichia coli

T7 development in Escherichia coli requires the inhibition of the housekeeping form of the bacterial RNA polymerase (RNAP), Eσ⁷⁰, by two T7 proteins: Gp2 and Gp5.7. Although the biological role of Gp2 is well understood, that of Gp5.7 remains to be fully deciphered. Here, we present results from functional and structural analyses to reveal that Gp5.7 primarily serves to inhibit Eσ^S, the predominant form of the RNAP in the stationary phase of growth, which accumulates in exponentially growing E. coli as a consequence of the buildup of guanosine pentaphosphate [(p)ppGpp] during T7 development. We further demonstrate a requirement of Gp5.7 for T7 development in E. coli cells in the stationary phase of growth. Our finding represents a paradigm for how some lytic phages have evolved distinct mechanisms to inhibit the bacterial transcription machinery to facilitate phage development in bacteria in the exponential and stationary phases of growth.

A non-canonical multisubunit RNA polymerase encoded by the AR9 phage recognizes the template strand of its uracil-containing promoters

AR9 is a giant Bacillus subtilis phage whose uracil-containing double-stranded DNA genome encodes distant homologs of β and β’ subunits of bacterial RNA polymerase (RNAP). The products of these genes are thought to assemble into two non-canonical multisubunit RNAPs - a virion RNAP (vRNAP) that is injected into the host along with phage DNA to transcribe early phage genes, and a non-virion RNAP (nvRNAP), which is synthesized during the infection and transcribes late phage genes. We purified the AR9 nvRNAP from infected B. subtilis cells and characterized its transcription activity in vitro. The AR9 nvRNAP requires uracils rather than thymines at specific conserved positions of late viral promoters. Uniquely, the nvRNAP recognizes the template strand of its promoters and is capable of specific initiation of transcription from both double- and single-stranded DNA. While the AR9 nvRNAP does not contain homologs of bacterial RNAP α subunits, it contains, in addition to the β and β’-like subunits, a phage protein gp226. The AR9 nvRNAP lacking gp226 is catalytically active but unable to bind to promoter DNA. Thus, gp226 is required for promoter recognition by the AR9 nvRNAP and may represent a new group of transcription initiation factors.

Promoter Recognition by Extracytoplasmic Function σ Factors: Analyzing DNA and Protein Interaction Motifs

Extracytoplasmic function (ECF) σ factors are the largest and the most diverse group of alternative σ factors, but their mechanisms of transcription are poorly studied. This subfamily is considered to exhibit a rigid promoter structure and an absence of mixing and matching; both -35 and -10 elements are considered necessary for initiating transcription. This paradigm, however, is based on very limited data, which bias the analysis of diverse ECF σ subgroups. Here we investigate DNA and protein recognition motifs involved in ECF σ factor transcription by a computational analysis of canonical ECF subfamily members, much less studied ECF σ subgroups, and the group outliers, obtained from recently sequenced bacteriophages. The analysis identifies an extended -10 element in promoters for phage ECF σ factors; a comparison with bacterial σ factors points to a putative 6-amino-acid motif just C-terminal of domain σ2, which is responsible for the interaction with the identified extension of the -10 element. Interestingly, a similar protein motif is found C-terminal of domain σ2 in canonical ECF σ factors, at a position where it is expected to interact with a conserved motif further upstream of the -10 element. Moreover, the phiEco32 ECF σ factor lacks a recognizable -35 element and σ4 domain, which we identify in a homologous phage, 7-11, indicating that the extended -10 element can compensate for the lack of -35 element interactions. Overall, the results reveal greater flexibility in promoter recognition by ECF σ factors than previously recognized and raise the possibility that mixing and matching also apply to this group, a notion that remains to be biochemically tested.

IMPORTANCE

ECF σ factors are the most numerous group of alternative σ factors but have been little studied. Their promoter recognition mechanisms are obscured by the large diversity within the ECF σ factor group and the limited similarity with the well-studied housekeeping σ factors. Here we extensively compare bacterial and bacteriophage ECF σ factors and their promoters in order to infer DNA and protein recognition motifs involved in transcription initiation. We predict a more flexible promoter structure than is recognized by the current paradigm, which assumes rigidness, and propose that ECF σ promoter elements may complement (mix and match with) each other's strengths. These results warrant the refocusing of research efforts from the well-studied housekeeping σ factors toward the physiologically highly important, but insufficiently understood, alternative σ factors.

Bacterial sigma factors: a historical, structural, and genomic perspective

Transcription initiation is the crucial focal point of gene expression in prokaryotes. The key players in this process, sigma factors (σs), associate with the catalytic core RNA polymerase to guide it through the essential steps of initiation: promoter recognition and opening, and synthesis of the first few nucleotides of the transcript. Here we recount the key advances in σ biology, from their discovery 45 years ago to the most recent progress in understanding their structure and function at the atomic level. Recent data provide important structural insights into the mechanisms whereby σs initiate promoter opening. We discuss both the housekeeping σs, which govern transcription of the majority of cellular genes, and the alternative σs, which direct RNA polymerase to specialized operons in response to environmental and physiological cues. The review concludes with a genome-scale view of the extracytoplasmic function σs, the most abundant group of alternative σs.

Distinct functions of the RNA polymerase σ subunit region 3.2 in RNA priming and promoter escape

The σ subunit of bacterial RNA polymerase (RNAP) has been implicated in all steps of transcription initiation, including promoter recognition and opening, priming of RNA synthesis, abortive initiation and promoter escape. The post-promoter-recognition σ functions were proposed to depend on its conserved region σ3.2 that directly contacts promoter DNA immediately upstream of the RNAP active centre and occupies the RNA exit path. Analysis of the transcription effects of substitutions and deletions in this region in Escherichia coli σ⁷⁰ subunit, performed in this work, suggests that (i) individual residues in the σ3.2 finger collectively contribute to RNA priming by RNAP, likely by the positioning of the template DNA strand in the active centre, but are not critical to promoter escape; (ii) the physical presence of σ3.2 in the RNA exit channel is important for promoter escape; (iii) σ3.2 promotes σ dissociation during initiation and suppresses σ-dependent promoter-proximal pausing; (iv) σ3.2 contributes to allosteric inhibition of the initiating NTP binding by rifamycins. Thus, region σ3.2 performs distinct functions in transcription initiation and its inhibition by antibiotics. The B-reader element of eukaryotic factor TFIIB likely plays similar roles in RNAPII transcription, revealing common principles in transcription initiation in various domains of life.

Specific contacts of the -35 region of the galP1 promoter by RNA polymerase inhibit GalR-mediated DNA looping repression

The P1 promoter of the galactose operon in Escherichia coli is one of the best studied examples of ‘extended −10’ promoters. Recognition of the P1 promoter does not require specific contacts between RNA polymerase and its poor −35 element. To investigate whether specific recognition of the −35 element would affect the regulation of P1 by GalR, we mutagenized the −35 element of P1, isolated variants of the −35 element and studied the regulation of the mutant promoters by in vitro transcription assays and by mathematical modeling. The results show that the GalR-mediated DNA loop is less efficient in repressing P1 transcription when RNA polymerase binds to the −10 and −35 elements concomitantly. Our results suggest that promoters that lack specific −35 element recognition allow decoupling of local chromosome structure from transcription initiation.

Redefining Escherichia coli σ(70) promoter elements: -15 motif as a complement of the -10 motif

Classical elements of σ(70) bacterial promoters include the -35 element ((-35)TTGACA(-30)), the -10 element ((-12)TATAAT(-7)), and the extended -10 element ((-15)TG(-14)). Although the -35 element, the extended -10 element, and the upstream-most base in the -10 element ((-12)T) interact with σ(70) in double-stranded DNA (dsDNA) form, the downstream bases in the -10 motif ((-11)ATAAT(-7)) are responsible for σ(70)-single-stranded DNA (ssDNA) interactions. In order to directly reflect this correspondence, an extension of the extended -10 element to a so-called -15 element ((-15)TGnT(-12)) has been recently proposed. I investigated here the sequence specificity of the proposed -15 element and its relationship to other promoter elements. I found a previously undetected significant conservation of (-13)G and a high degeneracy at (-15)T. I therefore defined the -15 element as a degenerate motif, which, together with the conserved stretch of sequence between -15 and -12, allows treating this element analogously to -35 and -10 elements. Furthermore, the strength of the -15 element inversely correlates with the strengths of the -35 element and -10 element, whereas no such complementation between other promoter elements was found. Despite the direct involvement of -15 element in σ(70)-dsDNA interactions, I found a significantly stronger tendency of this element to complement weak -10 elements that are involved in σ(70)-ssDNA interactions. This finding is in contrast to the established view, according to which the -15 element provides a sufficient number of σ(70)-dsDNA interactions, and suggests that the main parameter determining a functional promoter is the overall promoter strength.

Transcription, processing and function of CRISPR cassettes in Escherichia coli

CRISPR/Cas, bacterial and archaeal systems of interference with foreign genetic elements such as viruses or plasmids, consist of DNA loci called CRISPR cassettes (a set of variable spacers regularly separated by palindromic repeats) and associated cas genes. When a CRISPR spacer sequence exactly matches a sequence in a viral genome, the cell can become resistant to the virus. The CRISPR/Cas systems function through small RNAs originating from longer CRISPR cassette transcripts. While laboratory strains of Escherichia coli contain a functional CRISPR/Cas system (as judged by appearance of phage resistance at conditions of artificial co-overexpression of Cas genes and a CRISPR cassette engineered to target a λ-phage), no natural phage resistance due to CRISPR system function was observed in this best-studied organism and no E. coli CRISPR spacer matches sequences of well-studied E. coli phages. To better understand the apparently 'silent'E. coli CRISPR/Cas system, we systematically characterized processed transcripts from CRISPR cassettes. Using an engineered strain with genomically located spacer matching phage λ we show that endogenous levels of CRISPR cassette and cas genes expression allow only weak protection against infection with the phage. However, derepression of the CRISPR/Cas system by disruption of the hns gene leads to high level of protection.

Reduced capacity of alternative sigmas to melt promoters ensures stringent promoter recognition

In bacteria, multiple sigmas direct RNA polymerase to distinct sets of promoters. Housekeeping sigmas direct transcription from thousands of promoters, whereas most alternative sigmas are more selective, recognizing more highly conserved promoter motifs. For sigma(32) and sigma(28), two Escherichia coli Group 3 sigmas, altering a few residues in Region 2.3, the portion of sigma implicated in promoter melting, to those universally conserved in housekeeping sigmas relaxed their stringent promoter requirements and significantly enhanced melting of suboptimal promoters. All Group 3 sigmas and the more divergent Group 4 sigmas have nonconserved amino acids at these positions and rarely transcribe >100 promoters. We suggest that the balance of "melting" and "recognition" functions of sigmas is critical to setting the stringency of promoter recognition. Divergent sigmas may generally use a nonoptimal Region 2.3 to increase promoter stringency, enabling them to mount a focused response to altered conditions.

6S RNA binding to Esigma(70) requires a positively charged surface of sigma(70) region 4.2

6S RNA is a small, non-coding RNA that interacts with sigma(70)-RNA polymerase and downregulates transcription at many promoters during stationary phase. When bound to sigma(70)-RNA polymerase, 6S RNA is engaged in the active site of sigma(70)-RNA polymerase in a manner similar enough to promoter DNA that the RNA can serve as a template for RNA synthesis. It has been proposed that 6S RNA mimics the conformation of DNA during transcription initiation, suggesting contacts between RNA polymerase and 6S RNA or DNA may be similar. Here we demonstrate that region 4.2 of sigma(70) is critical for the interaction between 6S RNA and RNA polymerase. We define an expanded binding surface that encompasses positively charged residues throughout the recognition helix of the helix-turn-helix motif in region 4.2, in contrast to DNA binding that is largely focused on the N-terminal region of this helix. Furthermore, negatively charged residues in region 4.2 weaken binding to 6S RNA but do not similarly affect DNA binding. We propose that the binding sites for promoter DNA and 6S RNA on region 4.2 of sigma(70) are overlapping but distinct, raising interesting possibilities for how core promoter elements contribute to defining promoters that are sensitive to 6S RNA regulation.

Dissection of recognition determinants of Escherichia coli sigma32 suggests a composite -10 region with an 'extended -10' motif and a core -10 element

Sigma32 controls expression of heat shock genes in Escherichia coli and is widely distributed in proteobacteria. The distinguishing feature of sigma32 promoters is a long -10 region (CCCCATNT) whose tetra-C motif is important for promoter activity. Using alanine-scanning mutagenesis of sigma32 and in vivo and in vitro assays, we identified promoter recognition determinants of this motif. The most downstream C (-13) is part of the -10 motif; our work confirms and extends recognition determinants of -13C. Most importantly, our work suggests that the two upstream Cs (-16, -15) constitute an 'extended -10' recognition motif that is recognized by K130, a residue universally conserved in beta- and gamma-proteobacteria. This residue is located in the alpha-helix of sigmaDomain 3 that mediates recognition of the extended -10 promoter motif in other sigmas. K130 is not conserved in alpha- and delta-/epsilon-proteobacteria and we found that sigma32 from the alpha-proteobacterium Caulobacter crescentus does not need the extended -10 motif for high promoter activity. This result supports the idea that K130 mediates extended -10 recognition. Sigma32 is the first Group 3 sigma shown to use the 'extended -10' recognition motif.

Structural modules of RNA polymerase required for transcription from promoters containing downstream basal promoter element GGGA

We recently described a novel basal bacterial promoter element that is located downstream of the -10 consensus promoter element and is recognized by region 1.2 of the sigma subunit of RNA polymerase (RNAP). In the case of Thermus aquaticus RNAP, this element has a consensus sequence GGGA and allows transcription initiation in the absence of the -35 element. In contrast, the Escherichia coli RNAP is unable to initiate transcription from GGGA-containing promoters that lack the -35 element. In the present study, we demonstrate that sigma subunits from both E. coli and T. aquaticus specifically recognize the GGGA element and that the observed species specificity of recognition of GGGA-containing promoters is determined by the RNAP core enzyme. We further demonstrate that transcription initiation by T. aquaticus RNAP on GGGA-containing promoters in the absence of the -35 element requires sigma region 4 and C-terminal domains of the alpha subunits, which interact with upstream promoter DNA. When in the context of promoters containing the -35 element, the GGGA element is recognized by holoenzyme RNAPs from both E. coli and T. aquaticus and increases stability of promoter complexes formed on these promoters. Thus, GGGA is a bona fide basal promoter element that can function in various bacteria and, depending on the properties of the RNAP core enzyme and the presence of additional promoter elements, determine species-specific differences in promoter recognition.

Rapid isolation and identification of bacteriophage T4-encoded modifications of Escherichia coli RNA polymerase: a generic method to study bacteriophage/host interactions

Bacteriophages are bacterial viruses that infect bacterial cells, and they have developed ingenious mechanisms to modify the bacterial RNA polymerase. Using a rapid, specific, single-step affinity isolation procedure to purify Escherichia coli RNA polymerase from bacteriophage T4-infected cells, we have identified bacteriophage T4-dependent modifications of the host RNA polymerase. We suggest that this methodology is broadly applicable for the identification of bacteriophage-dependent alterations of the host synthesis machinery.

DNA bending in transcription initiation

Electrophoretic mobility shift (bandshift) phasing analysis and rotational variant topological analysis were performed on initiation complexes formed on the bacteriophage lambda PR promoter. Both the open complex and an abortive complex containing a short RNA primer extending to +3 were characterized. The two methods were used to analyze a series of constructs containing tandemly repeated copies of the PR promoter, with the repeat length increased in single base pair increments to progressively change the rotational setting of adjacent copies. The phasing effect observed in bandshift analysis of open complexes formed on this set of constructs provided qualitative evidence for the presence of a bend. Subsequent rotational variant topological analysis confirmed this and quantified the overall bend angle in the open complex as well as in the +3 abortive complex: a bend of 49 degrees +/- 7 degrees was measured for the open complex, while a bend of 47 degrees +/- 11 degrees was measured for the +3 complex, i.e., the two bends are the same. However, the topological results are not consistent with extensive superhelical wrapping of DNA on either complex as has been proposed. The two complexes do differ in the size of the transcription bubble: the open complex contains a 10.4 +/- 0.1 bp bubble, while that of the +3 complex is 12.2 +/- 0.1 bp, a result consistent with "DNA scrunching" during the onset of transcription. A model for the overall path of the DNA in the open complex is presented that is consistent with the measured bend angle. Measurement of both bubble size and overall bend angle complements the results of crystal structures in providing an enhanced description of the solution structures of the intact initiation complexes.

Promoter specificity for 6S RNA regulation of transcription is determined by core promoter sequences and competition for region 4.2 of sigma70

6S RNA binds sigma70-RNA polymerase and downregulates transcription at many sigma70-dependent promoters, but others escape regulation even during stationary phase when the majority of the transcription machinery is bound by the RNA. We report that core promoter elements determine this promoter specificity; a weak -35 element allows a promoter to be 6S RNA sensitive, and an extended -10 element similarly determines 6S RNA inhibition except when a consensus -35 element is present. These two features together predicted that hundreds of mapped Escherichia coli promoters might be subject to 6S RNA dampening in stationary phase. Microarray analysis confirmed 6S RNA-dependent downregulation of expression from 68% of the predicted genes, which corresponds to 49% of the expressed genes containing mapped E. coli promoters and establishes 6S RNA as a global regulator in stationary phase. We also demonstrate a critical role for region 4.2 of sigma70 in RNA polymerase interactions with 6S RNA. Region 4.2 binds the -35 element during transcription initiation; therefore we propose one mechanism for 6S RNA regulation of transcription is through competition for binding region 4.2 of sigma70.

Middle promoters constitute the most abundant and diverse class of promoters in bacteriophage T4

The temporally regulated transcription program of bacteriophage T4 relies upon the sequential utilization of three classes of promoters: early, middle and late. Here we show that middle promoters constitute perhaps the largest and the most diverse class of T4 promoters. In addition to 45 T4 middle promoters known to date, we mapped 13 new promoters, 10 of which deviate from the consensus MotA box, with some of them having no obvious match to it. So, 30 promoters of 58 identified now deviate from the consensus sequence deduced previously. In spite of the differences in their sequences, the in vivo activities of these T4 middle promoters were demonstrated to be dependent on both activators, MotA and AsiA. Traditionally, the MotA box was restricted to a 9 bp sequence with the highly conserved motif TGCTT. New logo based on the sequences of currently known middle promoters supports the conclusion that the consensus MotA box is comprised of 10 bp with the highly conserved central motif GCT. However, some apparently good matches to the consensus of middle promoters do not produce transcripts either in vivo or in vitro, indicating that the consensus sequence alone does not fully define a middle promoter.

6S RNA regulation of pspF transcription leads to altered cell survival at high pH

6S RNA is a highly abundant small RNA that regulates transcription through direct interaction with RNA polymerase. Here we show that 6S RNA directly inhibits transcription of pspF, which subsequently leads to inhibition of pspABCDE and pspG expression. Cells without 6S RNA are able to survive at elevated pH better than wild-type cells due to loss of 6S RNA-regulation of pspF. This 6S RNA-dependent phenotype is eliminated in pspF-null cells, indicating that 6S RNA effects are conferred through PspF. Similar growth phenotypes are seen when PspF levels are increased in a 6S RNA-independent manner, signifying that changes to pspF expression are sufficient. Changes in survival at elevated pH most likely result from altered expression of pspABCDE and/or pspG, both of which require PspF for transcription and are indirectly regulated by 6S RNA. 6S RNA provides another layer of regulation in response to high pH during stationary phase. We propose that the normal role of 6S RNA at elevated pH is to limit the extent of the psp response under conditions of nutrient deprivation, perhaps facilitating appropriate allocation of diminishing resources.

Region 3.2 of the sigma subunit contributes to the binding of the 3'-initiating nucleotide in the RNA polymerase active center and facilitates promoter clearance during initiation

Region 3.2 of the RNA polymerase sigma subunit forms a loop that protrudes toward RNA polymerase active center and partially blocks RNA exit channel. To provide some insights into the functional role of this region, we studied a deletion variant of the Escherichia coli sigma(70) subunit that lacked amino acids 513-519 corresponding to the tip of the loop. The deletion had multiple effects on transcription initiation including: (i) a significant decrease in the amount of short abortive RNAs synthesized during initiation, (ii) defects in promoter escape, (iii) loss of the contacts between the sigma subunit and the nascent RNA during initiation and, finally, (iv) dramatic increase in the K(m) value for the 3'-initiating nucleotide. At the same time, the mutation did not impair promoter opening and the binding of the 5'-initiating purine nucleotide. In summary, our data demonstrate an important role of sigma region 3.2 in the binding of initiating substrates in RNA polymerase active center and in the process of promoter clearance.

Transcription regulation by bacteriophage T4 AsiA

Bacteriophage T4 AsiA, a strong inhibitor of bacterial RNA polymerase, was the first antisigma protein to be discovered. Recent advances that made it possible to purify large amounts of this highly toxic protein led to an increased understanding of AsiA function and structure. In this review, we discuss how the small 10-KDa AsiA protein plays a key role in T4 development through its ability to both inhibit and activate bacterial RNA polymerase transcription.

Real-time characterization of intermediates in the pathway to open complex formation by Escherichia coli RNA polymerase at the T7A1 promoter

We have used time-resolved x-ray-generated hydroxyl radical footprinting to directly characterize, at single-nucleotide resolution, several intermediates in the pathway to open complex formation by Escherichia coli RNA polymerase on the T7A1 promoter at 37 degrees C. Three sets of intermediates, corresponding to two major conformational changes, are resolved as a function of time; multiple conformations equilibrate amongst each other before the next large structural change. Analysis of these data in the context of published structural models indicates that initial recognition involves interaction of the UP element with the alpha-subunit C-terminal domain and binding of the sigma subunit to the -35 sequence. In the subsequent isomerization step, two complexes with footprints extending into the -10 region can be differentiated as the DNA becomes distorted during nucleation of strand separation. During the final isomerization step, the downstream double helix becomes embedded in the beta/beta' jaws, leading to a transcriptionally active complex.

The strong efficiency of the Escherichia coli gapA P1 promoter depends on a complex combination of functional determinants

The Escherichia coli multi-promoter region of the gapA gene ensures a high level of GAPDH (glyceraldehyde-3-phosphate dehydrogenase) production under various growth conditions. In the exponential phase of growth, gapA mRNAs are mainly initiated at the highly efficient gapA P1 promoter. In the present study, by using site-directed mutagenesis and chemical probing of the RPo (open complex) formed by Esigma70 (holoenzyme associated with sigma70) RNAP (RNA polymerase) at promoter gapA P1, we show that this promoter is an extended -10 promoter that needs a -35 sequence for activity. The -35 sequence compensates for the presence of a suboptimal -10 hexamer. A tract of thymine residues in the spacer region, which is responsible for a DNA distortion, is also required for efficient activity. We present the first chemical probing of an RPo formed at a promoter needing both a -10 extension and a -35 sequence. It reveals a complex array of RNAP-DNA interactions. In agreement with the fact that residue A-11 in the non-template strand is flipped out in a protein pocket in previously studied RPos, the corresponding A residue in gapA P1 promoter is protected in RPo and is essential for activity. However, in contrast with some of the previous findings on RPos formed at other promoters, the -12 A:T pair is opened. Strong contacts with RNAP occur both with the -35 sequence and the TG extension, so that the sigma4 and sigma2 domains may simultaneously contact the promoter DNA. RNAP-DNA interactions were also detected immediately downstream of the -35 hexamer and in a more distal upstream segment, reflecting a wrapping of RNAP by the core and upstream promoter DNA. Altogether, the data reveal that promoter gapA P1 is a very efficient promoter sharing common properties with both extended -10 and non-extended -10 promoters.

The interaction between sigma70 and the beta-flap of Escherichia coli RNA polymerase inhibits extension of nascent RNA during early elongation

The sigma-subunit of bacterial RNA polymerase (RNAP) is required for promoter-specific transcription initiation. This function depends on specific intersubunit interactions that occur when sigma associates with the RNAP core enzyme to form RNAP holoenzyme. Among these interactions, that between conserved region 4 of sigma and the flap domain of the RNAP beta-subunit (beta-flap) is critical for recognition of the major class of bacterial promoters. Here, we describe the isolation of amino acid substitutions in region 4 of Escherichia coli sigma(70) that have specific effects on the sigma(70) region 4/beta-flap interaction, either weakening or strengthening it. Using these sigma(70) mutants, we demonstrate that the sigma region 4/beta-flap interaction also can affect events occurring downstream of transcription initiation during early elongation. Specifically, our results provide support for a structure-based proposal that, when bound to the beta-flap, sigma region 4 presents a barrier to the extension of the nascent RNA as it emerges from the RNA exit channel. Our findings support the view that the transition from initiation to elongation involves a staged disruption of sigma-core interactions.

Nature of the promoter activated by C.PvuII, an unusual regulatory protein conserved among restriction-modification systems

A widely distributed family of small regulators, called C proteins, controls a subset of restriction-modification systems. The C proteins studied to date activate transcription of their own genes and that of downstream endonuclease genes; this arrangement appears to delay endonuclease expression relative to that of the protective methyltransferase when the genes enter a new cell. C proteins bind to conserved sequences called C boxes. In the PvuII system, the C boxes have been reported to extend from -23 to +3 relative to the transcription start for the gene for the C protein, an unexpected starting position relative to a bound activator. This study suggests that transcript initiation within the C boxes represents initial, C-independent transcription of pvuIICR. The major C protein-dependent transcript appears to be a leaderless mRNA starting farther downstream, at the initiation codon for the pvuIIC gene. This conclusion is based on nuclease S1 transcript mapping and the effects of a series of nested deletions in the promoter region. Furthermore, replacing the region upstream of the pvuIIC initiation codon with a library of random oligonucleotides, followed by selection for C-dependent transcription, yielded clones having sequences that resemble -10 promoter hexamers. The -35 hexamer of this promoter would lie within the C boxes. However, the spacing between C boxes/-35 and the apparent -10 hexamer can be varied by +/-4 bp with little effect. This suggests that, like some other activator-dependent promoters, PpvuIICR may not require a -35 hexamer. Features of this transcription activation system suggest explanations for its broad host range.

The effects of upstream DNA on open complex formation by Escherichia coli RNA polymerase

Binding of activators to upstream DNA sequences regulates transcription initiation by affecting the stability of the initial RNA polymerase (RNAP)-promoter complex and/or the rate of subsequent conformational changes required to form the open complex (RP(O)). Here we observe that the presence of nonspecific upstream DNA profoundly affects an early step in formation of the transcription bubble. Kinetic studies with the lambdaP(R) promoter and Escherichia coli RNAP reveal that the presence of DNA upstream of base pair -47 greatly increases the rate of forming RP(O), without significantly affecting its rate of dissociation. We find that this increase is largely due to an acceleration of the rate-limiting step (isomerization) in RP(O) formation, a step that occurs after polymerase binds. Footprinting experiments reveal striking structural differences downstream of the transcription start site (+1) in the first kinetically significant intermediate when upstream DNA is present. On the template strand, the DNase I downstream boundary of this early intermediate is +20 when upstream DNA is present but is shortened by approximately two helical turns when upstream DNA beyond -47 is removed. KMnO(4) footprinting reveals an identical initiation bubble (-11 to +2), but unusual reactivity of template strand upstream cytosines (-12, -14, and -15) on the truncated promoter. Based on this work, we propose that early wrapping interactions between upstream DNA and the polymerase exterior strongly affect the events that control entry and subsequent unwinding of the DNA start site in the jaws of polymerase.

Minimal machinery of RNA polymerase holoenzyme sufficient for promoter melting

We determined the minimal portion of Escherichia coli RNA polymerase (RNAP) holoenzyme able to accomplish promoter melting, the crucial step in transcription initiation that provides RNAP access to the template strand. Upon duplex DNA binding, the N terminus of the beta' subunit (amino acids 1 to 314) and amino acids 94 to 507 of the sigma subunit, together comprising less than one-fifth of RNAP holoenzyme, were able to melt an extended -10 promoter in a reaction remarkably similar to that of authentic holoenzyme. Our results support the model that capture of nontemplate bases extruded from the DNA helix underlies the melting process.

Regulation of RNA polymerase promoter selectivity by covalent modification of DNA

Expression of genes encoding type II restriction/modification (R/M) systems, which are widely spread in eubacteria, must be tightly regulated to ensure that host DNA is protected from restriction endonucleases at all times. Examples of coordinated expression of R/M genes that rely on the action of regulatory factors or the ability of methyl transferases to repress their own synthesis by interacting with the promoter DNA have been described. Here, we characterize the molecular mechanism of factor-independent regulation in the CfrBI R/M system. Regulation of the cfrBIM gene transcription occurs through CfrBIM-catalyzed methylation of a cytosine residue in the cfrBIM promoter. The covalent modification inhibits cfrB1M promoter complex formation by interfering with the RNA polymerase sigma(70) subunit region 4.2 recognition of the -35 promoter element. The decrease in the cfrBIM promoter complex formation leads to increase in the activity of overlapping cfrBIR promoters. This elegant factor-independent regulatory system ensures coordinated expression of the cfrBI genes.