Bacteriophage T4 middle transcription system: T4-modified RNA polymerase; AsiA, a sigma 70 binding protein; and transcriptional activator MotA

An OmpW-dependent T4-like phage infects Serratia sp. ATCC 39006

Serratia sp. ATCC 39006 is a Gram-negative bacterium that has been used to study the function of phage defences, such as CRISPR-Cas, and phage counter-defence mechanisms. To expand our phage collection to study the phage-host interaction with Serratia sp. ATCC 39006, we isolated the T4-like myovirus LC53 in Ōtepoti Dunedin, Aotearoa New Zealand. Morphological, phenotypic and genomic characterization revealed that LC53 is virulent and similar to other Serratia, Erwinia and Kosakonia phages belonging to the genus Winklervirus. Using a transposon mutant library, we identified the host ompW gene as essential for phage infection, suggesting that it encodes the phage receptor. The genome of LC53 encodes all the characteristic T4-like core proteins involved in phage DNA replication and generation of viral particles. Furthermore, our bioinformatic analysis suggests that the transcriptional organization of LC53 is similar to that of Escherichia coli phage T4. Importantly, LC53 encodes 18 tRNAs, which likely compensate for differences in GC content between phage and host genomes. Overall, this study describes a newly isolated phage infecting Serratia sp. ATCC 39006 that expands the diversity of phages available to study phage-host interactions.

The σ Subunit-Remodeling Factors: An Emerging Paradigms of Transcription Regulation

Transcription initiation is a key checkpoint and highly regulated step of gene expression. The sigma (σ) subunit of RNA polymerase (RNAP) controls all transcription initiation steps, from recognition of the -10/-35 promoter elements, upon formation of the closed promoter complex (RPc), to stabilization of the open promoter complex (RPo) and stimulation of the primary steps in RNA synthesis. The canonical mechanism to regulate σ activity upon transcription initiation relies on activators that recognize specific DNA motifs and recruit RNAP to promoters. This mini-review describes an emerging group of transcriptional regulators that form a complex with σ or/and RNAP prior to promoter binding, remodel the σ subunit conformation, and thus modify RNAP activity. Such strategy is widely used by bacteriophages to appropriate the host RNAP. Recent findings on RNAP-binding protein A (RbpA) from Mycobacterium tuberculosis and Crl from Escherichia coli suggest that activator-driven changes in σ conformation can be a widespread regulatory mechanism in bacteria.

Bacteriophage gene products as potential antimicrobials against tuberculosis

Tuberculosis (TB) is recognised as one of the most pressing global health threats among infectious diseases. Bacteriophages are adapted for killing of their host, and they were exploited in antibacterial therapy already before the discovery of antibiotics. Antibiotics as broadly active drugs overshadowed phage therapy for a long time. However, owing to the rapid spread of antibiotic resistance and the increasing complexity of treatment of drug-resistant TB, mycobacteriophages are being studied for their antimicrobial potential. Besides phage therapy, which is the administration of live phages to infected patients, the development of drugs of phage origin is gaining interest. This path of medical research might provide us with a new pool of previously undiscovered inhibition mechanisms and molecular interactions which are also of interest in basic research of cellular processes, such as transcription. The current state of research on mycobacteriophage-derived anti-TB treatment is reviewed in comparison with inhibitors from other phages, and with focus on transcription as the host target process.

The E. coli Global Regulator DksA Reduces Transcription during T4 Infection

Bacteriophage T4 relies on host RNA polymerase to transcribe three promoter classes: early (Pe, requires no viral factors), middle (Pm, requires early proteins MotA and AsiA), and late (Pl, requires middle proteins gp55, gp33, and gp45). Using primer extension, RNA-seq, RT-qPCR, single bursts, and a semi-automated method to document plaque size, we investigated how deletion of DksA or ppGpp, two E. coli global transcription regulators, affects T4 infection. Both ppGpp⁰ and ΔdksA increase T4 wild type (wt) plaque size. However, ppGpp⁰ does not significantly alter burst size or latent period, and only modestly affects T4 transcript abundance, while ΔdksA increases burst size (2-fold) without affecting latent period and increases the levels of several Pe transcripts at 5 min post-infection. In a T4motA^am infection, ΔdksA increases plaque size and shortens latent period, and the levels of specific middle RNAs increase due to more transcription from Pe’s that extend into these middle genes. We conclude that DksA lowers T4 early gene expression. Consequently, ΔdksA results in a more productive wt infection and ameliorates the poor expression of middle genes in a T4motA^am infection. As DksA does not inhibit Pe transcription in vitro, regulation may be indirect or perhaps requires additional factors.

Visualizing the phage T4 activated transcription complex of DNA and E. coli RNA polymerase

The ability of RNA polymerase (RNAP) to select the right promoter sequence at the right time is fundamental to the control of gene expression in all organisms. However, there is only one crystallized structure of a complete activator/RNAP/DNA complex. In a process called σ appropriation, bacteriophage T4 activates a class of phage promoters using an activator (MotA) and a co-activator (AsiA), which function through interactions with the σ⁷⁰ subunit of RNAP. We have developed a holistic, structure-based model for σ appropriation using multiple experimentally determined 3D structures (Escherichia coli RNAP, the Thermus aquaticus RNAP/DNA complex, AsiA /σ⁷⁰ Region 4, the N-terminal domain of MotA [MotA^NTD], and the C-terminal domain of MotA [MotA^CTD]), molecular modeling, and extensive biochemical observations indicating the position of the proteins relative to each other and to the DNA. Our results visualize how AsiA/MotA redirects σ, and therefore RNAP activity, to T4 promoter DNA, and demonstrate at a molecular level how the tactful interaction of transcriptional factors with even small segments of RNAP can alter promoter specificity. Furthermore, our model provides a rational basis for understanding how a mutation within the β subunit of RNAP (G1249D), which is far removed from AsiA or MotA, impairs σ appropriation.

Bordetella pertussis fim3 gene regulation by BvgA: phosphorylation controls the formation of inactive vs. active transcription complexes

Two-component systems [sensor kinase/response regulator (RR)] are major tools used by microorganisms to adapt to environmental conditions. RR phosphorylation is typically required for gene activation, but few studies have addressed how and if phosphorylation affects specific steps during transcription initiation. We characterized transcription complexes made with RNA polymerase and the Bordetella pertussis RR, BvgA, in its nonphosphorylated or phosphorylated (BvgA∼P) state at P(fim3), the promoter for the virulence gene fim3 (fimbrial subunit), using gel retardation, potassium permanganate and DNase I footprinting, cleavage reactions with protein conjugated with iron bromoacetamidobenzyl-EDTA, and in vitro transcription. Previous work has shown that the level of nonphosphorylated BvgA remains high in vivo under conditions in which BvgA is phosphorylated. Our results here indicate that surprisingly both BvgA and BvgA∼P form open and initiating complexes with RNA polymerase at P(fim3). However, phosphorylation of BvgA is needed to generate the correct conformation that can transition to competent elongation. Footprints obtained with the complexes made with nonphosphorylated BvgA are atypical; while the initiating complex with BvgA synthesizes short RNA, it does not generate full-length transcripts. Extended incubation of the BvgA/RNA polymerase initiated complex in the presence of heparin generates a stable, but defective species that depends on the initial transcribed sequence of fim3. We suggest that the presence of nonphosphorylated BvgA down-regulates P(fim3) activity when phosphorylated BvgA is present and may allow the bacterium to quickly adapt to the loss of inducing conditions by rapidly eliminating P(fim3) activation once the signal for BvgA phosphorylation is removed.

Evidence for sigma factor competition in the regulation of alginate production by Pseudomonas aeruginosa

Alginate overproduction, or mucoidy, plays an important role in the pathogenesis of P. aeruginosa lung infection in cystic fibrosis (CF). Mucoid strains with mucA mutations predominantly populate in chronically-infected patients. However, the mucoid strains can revert to nonmucoidy in vitro through suppressor mutations. We screened a mariner transposon library using CF149, a non-mucoid clinical isolate with a misssense mutation in algU (AlgU^A61V). The wild type AlgU is a stress-related sigma factor that activates transcription of alginate biosynthesis. Three mucoid mutants were identified with transposon insertions that caused 1) an overexpression of AlgU^A61V, 2) an overexpression of the stringent starvation protein A (SspA), and 3) a reduced expression of the major sigma factor RpoD (σ⁷⁰). Induction of AlgU^A61Vin trans caused conversion to mucoidy in CF149 and PAO1DalgU, suggesting that AlgU^A61V is functional in activating alginate production. Furthermore, the level of AlgU^A61V was increased in all three mutants relative to CF149. However, compared to the wild type AlgU, AlgU^A61V had a reduced activity in promoting alginate production in PAO1ΔalgU. SspA and three other anti-σ⁷⁰ orthologues, P. aeruginosa AlgQ, E. coli Rsd, and T4 phage AsiA, all induced mucoidy, suggesting that reducing activity of RpoD is linked to mucoid conversion in CF149. Conversely, RpoD overexpression resulted in suppression of mucoidy in all mucoid strains tested, indicating that sigma factor competition can regulate mucoidy. Additionally, an RpoD-dependent promoter (P_ssrA) was more active in non-mucoid strains than in isogenic mucoid variants. Altogether, our results indicate that the anti-σ⁷⁰ factors can induce conversion to mucoidy in P. aeruginosa CF149 with algU-suppressor mutation via modulation of RpoD.

Bacteriophage T4 MotA activator and the β-flap tip of RNA polymerase target the same set of σ70 carboxyl-terminal residues

Sigma factors, the specificity subunits of RNA polymerase, are involved in interactions with promoter DNA, the core subunits of RNA polymerase, and transcription factors. The bacteriophage T4-encoded activator, MotA, is one such factor, which engages the C terminus of the Escherichia coli housekeeping sigma factor, σ(70). MotA functions in concert with a phage-encoded co-activator, AsiA, as a molecular switch. This process, termed sigma appropriation, inhibits host transcription while activating transcription from a class of phage promoters. Previous work has demonstrated that MotA contacts the C terminus of σ(70), H5, a region that is normally bound within RNA polymerase by its interaction with the β-flap tip. To identify the specific σ(70) residues responsible for interacting with MotA and the β-flap tip, we generated single substitutions throughout the C terminus of σ(70). We find that MotA targets H5 residues that are normally engaged by the β-flap. In two-hybrid assays, the interaction of σ(70) with either the β-flap tip or MotA is impaired by alanine substitutions at residues Leu-607, Arg-608, Phe-610, Leu-611, and Asp-613. Transcription assays identify Phe-610 and Leu-611 as the key residues for MotA/AsiA-dependent transcription. Phe-610 is a crucial residue in the H5/β-flap tip interaction using promoter clearance assays with RNA polymerase alone. Our results show how the actions of small transcriptional factors on a defined local region of RNA polymerase can fundamentally change the specificity of polymerase.

Transcriptional control in the prereplicative phase of T4 development

Control of transcription is crucial for correct gene expression and orderly development. For many years, bacteriophage T4 has provided a simple model system to investigate mechanisms that regulate this process. Development of T4 requires the transcription of early, middle and late RNAs. Because T4 does not encode its own RNA polymerase, it must redirect the polymerase of its host, E. coli, to the correct class of genes at the correct time. T4 accomplishes this through the action of phage-encoded factors. Here I review recent studies investigating the transcription of T4 prereplicative genes, which are expressed as early and middle transcripts. Early RNAs are generated immediately after infection from T4 promoters that contain excellent recognition sequences for host polymerase. Consequently, the early promoters compete extremely well with host promoters for the available polymerase. T4 early promoter activity is further enhanced by the action of the T4 Alt protein, a component of the phage head that is injected into E. coli along with the phage DNA. Alt modifies Arg265 on one of the two α subunits of RNA polymerase. Although work with host promoters predicts that this modification should decrease promoter activity, transcription from some T4 early promoters increases when RNA polymerase is modified by Alt. Transcription of T4 middle genes begins about 1 minute after infection and proceeds by two pathways: 1) extension of early transcripts into downstream middle genes and 2) activation of T4 middle promoters through a process called sigma appropriation. In this activation, the T4 co-activator AsiA binds to Region 4 of σ⁷⁰, the specificity subunit of RNA polymerase. This binding dramatically remodels this portion of σ⁷⁰, which then allows the T4 activator MotA to also interact with σ⁷⁰. In addition, AsiA restructuring of σ⁷⁰ prevents Region 4 from forming its normal contacts with the -35 region of promoter DNA, which in turn allows MotA to interact with its DNA binding site, a MotA box, centered at the -30 region of middle promoter DNA. T4 sigma appropriation reveals how a specific domain within RNA polymerase can be remolded and then exploited to alter promoter specificity.

Determinants of affinity and activity of the anti-sigma factor AsiA

The AsiA protein is a T4 bacteriophage early gene product that regulates transcription of host and viral genes. Monomeric AsiA binds tightly to the sigma(70) subunit of Escherichia coli RNA polymerase, thereby inhibiting transcription from bacterial promoters and phage early promoters and coactivating transcription from phage middle promoters. Results of structural studies have identified amino acids at the protomer-protomer interface in dimeric AsiA and at the monomeric AsiA-sigma(70) interface and demonstrated substantial overlap in the sets of residues that comprise each. Here we evaluate the contributions of individual interfacial amino acid side chains to protomer-protomer affinity in AsiA homodimers, to monomeric AsiA affinity for sigma(70), and to AsiA function in transcription. Sedimentation equilibrium, dynamic light scattering, electrophoretic mobility shift, and transcription activity measurements were used to assess affinity and function of site-specific AsiA mutants. Alanine substitutions for solvent-inaccessible residues positioned centrally in the protomer-protomer interface of the AsiA homodimer, V14, I17, and I40, resulted in the largest changes in free energy of dimer association, whereas alanine substitutions at other interfacial positions had little effect. These residues also contribute significantly to AsiA-dependent regulation of RNA polymerase activity, as do additional residues positioned at the periphery of the interface (K20 and F21). Notably, the relative contributions of a given amino acid side chain to RNA polymerase inhibition and activation (MotA-independent) by AsiA are very similar in most cases. The mainstay for intermolecular affinity and AsiA function appears to be I17. Our results define the core interfacial residues of AsiA, establish roles for many of the interfacial amino acids, are in agreement with the tenets underlying protein-protein interactions and interfaces, and will be beneficial for a general, comprehensive understanding of the mechanistic underpinnings of bacterial RNA polymerase regulation.

A mutation within the β subunit of Escherichia coli RNA polymerase impairs transcription from bacteriophage T4 middle promoters

During infection of Escherichia coli, bacteriophage T4 usurps the host transcriptional machinery, redirecting it to the expression of early, middle, and late phage genes. Middle genes, whose expression begins about 1 min postinfection, are transcribed both from the extension of early RNA into middle genes and by the activation of T4 middle promoters. Middle-promoter activation requires the T4 transcriptional activator MotA and coactivator AsiA, which are known to interact with σ(70), the specificity subunit of RNA polymerase. T4 motA amber [motA(Am)] or asiA(Am) phage grows poorly in wild-type E. coli. However, previous work has found that T4 motA(Am)does not grow in the E. coli mutant strain TabG. We show here that the RNA polymerase in TabG contains two mutations within its β-subunit gene: rpoB(E835K) and rpoB(G1249D). We find that the G1249D mutation is responsible for restricting the growth of either T4 motA(Am)or asiA(Am) and for impairing transcription from MotA/AsiA-activated middle promoters in vivo. With one exception, transcription from tested T4 early promoters is either unaffected or, in some cases, even increases, and there is no significant growth phenotype for the rpoB(E835K G1249D) strain in the absence of T4 infection. In reported structures of thermophilic RNA polymerase, the G1249 residue is located immediately adjacent to a hydrophobic pocket, called the switch 3 loop. This loop is thought to aid in the separation of the RNA from the DNA-RNA hybrid as RNA enters the RNA exit channel. Our results suggest that the presence of MotA and AsiA may impair the function of this loop or that this portion of the β subunit may influence interactions among MotA, AsiA, and RNA polymerase.

A basic/hydrophobic cleft of the T4 activator MotA interacts with the C-terminus of E.coli sigma70 to activate middle gene transcription

Transcriptional activation often employs a direct interaction between an activator and RNA polymerase. For activation of its middle genes, bacteriophage T4 appropriates Escherichia coli RNA polymerase through the action of two phage-encoded proteins, MotA and AsiA. Alone, AsiA inhibits transcription from a large class of host promoters by structurally remodelling region 4 of sigma(70), the primary specificity subunit of E. coli RNA polymerase. MotA interacts both with sigma(70) region 4 and with a DNA element present in T4 middle promoters. AsiA-induced remodelling is proposed to make the far C-terminus of sigma(70) region 4 accessible for MotA binding. Here, NMR chemical shift analysis indicates that MotA uses a 'basic/hydrophobic' cleft to interact with the C-terminus of AsiA-remodelled sigma(70), but MotA does not interact with AsiA itself. Mutations within this cleft, at residues K3, K28 and Q76, both impair the interaction of MotA with sigma(70) region 4 and MotA-dependent activation. Furthermore, mutations at these residues greatly decrease phage viability. Most previously described activators that target sigma(70) directly use acidic residues to engage a basic surface of region 4. Our work supports accumulated evidence indicating that 'sigma appropriation' by MotA and AsiA uses a fundamentally different mechanism to activate transcription.

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.

Mutational analysis of sigma70 region 4 needed for appropriation by the bacteriophage T4 transcription factors AsiA and MotA

Transcriptional activation of bacteriophage T4 middle promoters requires sigma70-containing Escherichia coli RNA polymerase, the T4 activator MotA, and the T4 co-activator AsiA. T4 middle promoters contain the sigma70 -10 DNA element. However, these promoters lack the sigma70 -35 element, having instead a MotA box centered at -30, which is bound by MotA. Previous work has indicated that AsiA and MotA interact with region 4 of sigma70, the C-terminal portion that normally contacts -35 DNA and the beta-flap structure in core. AsiA binding prevents the sigma70/beta-flap and sigma70/-35 DNA interactions, inhibiting transcription from promoters that require a -35 element. To test the importance of residues within sigma70 region 4 for MotA and AsiA function, we investigated how sigma70 region 4 mutants interact with AsiA, MotA, and the beta-flap and function in transcription assays in vitro. We find that alanine substitutions at residues 584-588 (region 4.2) do not impair the interaction of region 4 with the beta-flap or MotA, but they eliminate the interaction with AsiA and prevent AsiA inhibition and MotA/AsiA activation. In contrast, alanine substitutions at 551-552, 554-555 (region 4.1) eliminate the region 4/beta-flap interaction, significantly impair the AsiA/sigma70 interaction, and eliminate AsiA inhibition. However, the 4.1 mutant sigma70 is still fully competent for activation if both MotA and AsiA are present. A previous NMR structure shows AsiA binding to sigma70 region 4, dramatically distorting regions 4.1 and 4.2 and indirectly changing the conformation of the MotA interaction site at the sigma70 C terminus. Our analyses provide biochemical relevance for the sigma70 residues identified in the structure, indicate that the interaction of AsiA with sigma70 region 4.2 is crucial for activation, and support the idea that AsiA binding facilitates an interaction between MotA and the far C terminus of sigma70.

The bacteriophage T4 inhibitor and coactivator AsiA inhibits Escherichia coli RNA Polymerase more rapidly in the absence of sigma70 region 1.1: evidence that region 1.1 stabilizes the interaction between sigma70 and core

The N-terminal region (region 1.1) of sigma70, the primary sigma subunit of Escherichia coli RNA polymerase, is a negatively charged domain that affects the DNA binding properties of sigma70 regions 2 and 4. Region 1.1 prevents the interaction of free sigma70 with DNA and modulates the formation of stable (open) polymerase/promoter complexes at certain promoters. The bacteriophage T4 AsiA protein is an inhibitor of sigma70-dependent transcription from promoters that require an interaction between sigma70 region 4 and the -35 DNA element and is the coactivator of transcription at T4 MotA-dependent promoters. Like AsiA, the T4 activator MotA also interacts with sigma70 region 4. We have investigated the effect of region 1.1 on AsiA inhibition and MotA/AsiA activation. We show that sigma70 region 1.1 is not required for MotA/AsiA activation at the T4 middle promoter P(uvsX). However, the rate of AsiA inhibition and of MotA/AsiA activation of polymerase is significantly increased when region 1.1 is missing. We also find that RNA polymerase reconstituted with sigma70 that lacks region 1.1 is less stable than polymerase with full-length sigma70. Our previous work has demonstrated that the AsiA-inhibited polymerase is formed when AsiA binds to region 4 of free sigma70 and then the AsiA/sigma70 complex binds to core. Our results suggest that in the absence of region 1.1, there is a shift in the dynamic equilibrium between polymerase holoenzyme and free sigma70 plus core, yielding more free sigma70 at any given time. Thus, the rate of AsiA inhibition and AsiA/MotA activation increases when RNA polymerase lacks region 1.1 because of the increased availability of free sigma70. Previous work has argued both for and against a direct interaction between regions 1.1 and 4. Using an E. coli two-hybrid assay, we do not detect an interaction between these regions. This result supports the idea that the ability of region 1.1 to prevent DNA binding by free sigma70 arises through an indirect effect.

An artificial activator that contacts a normally occluded surface of the RNA polymerase holoenzyme

Many activators of transcription are sequence-specific DNA-binding proteins that stimulate transcription initiation through interaction with RNA polymerase (RNAP). Such activators can be constructed artificially by fusing a DNA-binding protein to a protein domain that can interact with an accessible surface of RNAP. In these cases, the artificial activator is directed to a target promoter bearing a recognition site for the DNA-binding protein. Here we describe an artificial activator that functions by contacting a normally occluded surface of promoter-bound RNAP holoenzyme. This artificial activator consists of a DNA-binding protein fused to the bacteriophage T4-encoded transcription regulator AsiA. On its own, AsiA inhibits transcription by Escherichia coli RNAP because it remodels the holoenzyme, disrupting an intersubunit interaction that is required for recognition of the major class of bacterial promoters. However, when tethered to the DNA via a DNA-binding protein, AsiA can exert a strong stimulatory effect on transcription by disrupting the same intersubunit interaction, contacting an otherwise occluded surface of the holoenzyme. We show that mutations that affect the intersubunit interaction targeted by AsiA modulate the stimulatory effect of this artificial activator. Our results thus demonstrate that changes in the accessibility of a normally occluded surface of the RNAP holoenzyme can modulate the activity of a gene-specific regulator of transcription.

Transcriptional takeover by σ appropriation: remodelling of the σ 70 subunit of Escherichia coli RNA polymerase by the bacteriophage T4 activator MotA and co-activator AsiA

Activation of bacteriophage T4 middle promoters, which occurs about 1 min after infection, uses two phage-encoded factors that change the promoter specificity of the host RNA polymerase. These phage factors, the MotA activator and the AsiA co-activator, interact with theσ70specificity subunit ofEscherichia coliRNA polymerase, which normally contacts the −10 and −35 regions of host promoter DNA. Like host promoters, T4 middle promoters have a good match to the canonicalσ70DNA element located in the −10 region. However, instead of theσ70DNA recognition element in the promoter's −35 region, they have a 9 bp sequence (a MotA box) centred at −30, which is bound by MotA. Recent work has begun to provide information about the MotA/AsiA system at a detailed molecular level. Accumulated evidence suggests that the presence of MotA and AsiA reconfigures protein–DNA contacts in the upstream promoter sequences, without significantly affecting the contacts ofσ70with the −10 region. This type of activation, which is called ‘σappropriation’, is fundamentally different from other well-characterized models of prokaryotic activation in which an activator frequently serves to forceσ70to contact a less than ideal −35 DNA element. This review summarizes the interactions of AsiA and MotA withσ70, and discusses how these interactions accomplish the switch to T4 middle promoters by inhibiting the typical contacts of the C-terminal region ofσ70, region 4, with the host −35 DNA element and with other subunits of polymerase.

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.

Molecular gymnastics: distortion of an RNA polymerase σ factor

A recent structure obtained by nuclear magnetic resonance (NMR) spectroscopy shows that the binding of a small phage factor to the sigma(70) subunit of Escherichia coli RNA polymerase induces an unprecedented remodeling of a region of sigma(70), converting a DNA-binding helix-turn-helix into a continuous pseudohelix. This conformational change suggests how the phage factor can function both as an inhibitor and co-activator of transcription.

An altered-specificity DNA-binding mutant of Escherichia coliσ70 facilitates the analysis of σ70 function in vivo

The sigma subunit of bacterial RNA polymerase is strictly required for promoter recognition. The primary (housekeeping) sigma factor of Escherichia coli, sigma(70), is responsible for most of the gene expression in exponentially growing cells. The fact that sigma(70) is an essential protein has complicated efforts to genetically dissect the functions of sigma(70). To facilitate the analysis of sigma(70) function in vivo, we isolated an altered-specificity DNA-binding mutant of sigma(70), sigma(70) R584A, which preferentially recognizes a mutant promoter that is not efficiently recognized by wild-type sigma(70). Exploiting this sigma(70) mutant as a genetic tool, we establish an in vivo assay for the inhibitory effect of the bacteriophage T4-encoded anti-sigma factor AsiA on sigma(70)-dependent transcription. Our results demonstrate the utility of this altered-specificity system for genetically dissecting sigma(70) and its interactions with transcription regulators.

A family of anti-sigma70 proteins in T4-type phages and bacteria that are similar to AsiA, a Transcription inhibitor and co-activator of bacteriophage T4

Anti-sigma70 factors interact with sigma70 proteins, the specificity subunits of prokaryotic RNA polymerase. The bacteriophage T4 anti-sigma70 protein, AsiA, binds tightly to regions 4.1 and 4.2 of the sigma70 subunit of Escherichia coli RNA polymerase and inhibits transcription from sigma70 promoters that require recognition of the canonical sigma70 -35 DNA sequence. In the presence of the T4 transcription activator MotA, AsiA also functions as a co-activator of transcription from T4 middle promoters, which retain the canonical sigma70 -10 consensus sequence but have a MotA box sequence centered at -30 rather than the sigma70 -35 sequence. The E.coli anti-sigma70 protein Rsd also interacts with region 4.2 of sigma70 and inhibits transcription from sigma70 promoters. Our sequence comparisons of T4 AsiA with Rsd, with the predicted AsiA orthologs of the T4-type phages RB69, 44RR, KVP40, and Aeh1, and with AlgQ, a regulator of alginate production in Pseudomonas aeruginosa indicate that these proteins share conserved amino acid residues at positions known to be important for the binding of T4 AsiA to sigma70 region 4. We show that, like T4 AsiA, Rsd binds to sigma70 in a native protein gel and, as with T4 AsiA, a L18S substitution in Rsd disrupts this complex. Previous work has assigned sigma70 amino acid F563, within region 4.1, as a critical determinant for AsiA binding. This residue is also involved in the binding of sigma70 to the beta-flap of core, suggesting that AsiA inhibits transcription by disrupting the interaction between sigma70 region 4.1 and the beta-flap. We find that as with T4 AsiA, the interaction of KVP40 AsiA, Rsd, or AlgQ with sigma70 region 4 is diminished by the substitution F563Y. We also demonstrate that like T4 AsiA and Rsd, KVP40 AsiA inhibits transcription from sigma70-dependent promoters. We speculate that the phage AsiA orthologs, Rsd, and AlgQ are members of a related family in T4-type phage and bacteria, which interact similarly with primary sigma factors. In addition, we show that even though a clear MotA ortholog has not been identified in the KVP40 genome and the phage genome appears to lack typical middle promoter sequences, KVP40 AsiA activates transcription from T4 middle promoters in the presence of T4 MotA. We speculate that KVP40 encodes a protein that is dissimilar in sequence, but functionally equivalent, to T4 MotA.

Escherichia coli SspA is a transcription activator for bacteriophage P1 late genes

The stringent starvation protein A (SspA), an Escherichia coli RNA polymerase (RNAP)-associated protein, has been reported to be essential for lytic growth of bacteriophage P1. Unlike P1 early promoters, P1 late promoters are not recognized by RNAP alone. A phage-encoded early protein, Lpa (late promoter activator protein, formerly called gp10), has been shown to be required for P1 late transcription in vivo. Here, we demonstrate that SspA is a transcription activator for P1 late genes. Our results indicated that Lpa is not limiting in an sspA mutant. However, the transcription of P1 late genes was deficient in an sspA mutant in vivo. We demonstrated that SspA/Lpa are required for transcription activation of the P1 late promoter Ps in vitro. In addition, SspA and Lpa were shown to facilitate the binding of RNAP to Ps late promoter DNA. Activation of late transcription by SspA/Lpa was dependent on holoenzyme containing sigma70 but not sigmaS, indicating that the two activators discriminate between the two forms of the holoenzyme. Furthermore, P1 early gene expression was downregulated in the wild-type background, whereas it persisted in the sspA mutant background, indicating that SspA/Lpa mediate the transcriptional switch from the early to the late genes during P1 lytic growth. Thus, this work provides the first evidence for a function of the E. coli RNAP-associated protein SspA.

Bacteriophage T4 genome

Phage T4 has provided countless contributions to the paradigms of genetics and biochemistry. Its complete genome sequence of 168,903 bp encodes about 300 gene products. T4 biology and its genomic sequence provide the best-understood model for modern functional genomics and proteomics. Variations on gene expression, including overlapping genes, internal translation initiation, spliced genes, translational bypassing, and RNA processing, alert us to the caveats of purely computational methods. The T4 transcriptional pattern reflects its dependence on the host RNA polymerase and the use of phage-encoded proteins that sequentially modify RNA polymerase; transcriptional activator proteins, a phage sigma factor, anti-sigma, and sigma decoy proteins also act to specify early, middle, and late promoter recognition. Posttranscriptional controls by T4 provide excellent systems for the study of RNA-dependent processes, particularly at the structural level. The redundancy of DNA replication and recombination systems of T4 reveals how phage and other genomes are stably replicated and repaired in different environments, providing insight into genome evolution and adaptations to new hosts and growth environments. Moreover, genomic sequence analysis has provided new insights into tail fiber variation, lysis, gene duplications, and membrane localization of proteins, while high-resolution structural determination of the "cell-puncturing device," combined with the three-dimensional image reconstruction of the baseplate, has revealed the mechanism of penetration during infection. Despite these advances, nearly 130 potential T4 genes remain uncharacterized. Current phage-sequencing initiatives are now revealing the similarities and differences among members of the T4 family, including those that infect bacteria other than Escherichia coli. T4 functional genomics will aid in the interpretation of these newly sequenced T4-related genomes and in broadening our understanding of the complex evolution and ecology of phages-the most abundant and among the most ancient biological entities on Earth.

Interaction of T4 AsiA with its target sites in the RNA polymerase sigma70 subunit leads to distinct and opposite effects on transcription

Bacteriophage T4 AsiA is a homodimeric protein that orchestrates a switch from the host and early viral transcription to middle viral transcription by binding to the sigma(70) subunit of Escherichia coli RNA polymerase holoenzyme (Esigma(70)) and preventing promoter complex formation on most E.coli and early T4 promoters. In addition, Esigma(70)AsiA, but not Esigma(70), is a substrate of transcription activation by T4-encoded DNA-binding protein MotA, a co-activator of transcription from middle viral promoters. The molecular determinants of sigma(70)-AsiA interaction necessary for transcription inhibition reside in the sigma(70) conserved region 4.2, which recognizes the -35 promoter consensus element. The molecular determinants of sigma(70)-AsiA interaction necessary for MotA-dependent transcription activation have not been identified. Here, we show that in the absence of sigma(70) region 4.2, AsiA interacts with sigma(70) conserved region 4.1 and activates transcription in a MotA-independent manner. Further, we show that the AsiA dimer must dissociate to interact with either region 4.2 or region 4.1 of sigma(70). We propose that MotA may co-activate transcription by restricting AsiA binding to sigma(70) region 4.1.

Analysis of regions within the bacteriophage T4 AsiA protein involved in its binding to the sigma70 subunit of E. coli RNA polymerase and its role as a transcriptional inhibitor and co-activator

Bacteriophage T4 AsiA, a protein of 90 amino acid residues, binds to the sigma(70) subunit of Escherichia coli RNA polymerase and inhibits host or T4 early transcription or, together with the T4 MotA protein, activates T4 middle transcription. To investigate which regions within AsiA are involved in forming a complex with sigma(70) and in providing transcriptional functions we generated random mutations throughout AsiA and targeted mutations within the C-terminal region. We tested mutant proteins for their ability to complement the growth of T4 asiA am phage under non-suppressing conditions, to inhibit E. coli growth, to interact with sigma(70) region 4 in a two-hybrid assay, to bind to sigma(70) in a native protein gel, and to inhibit or activate transcription in vitro using a T4 middle promoter that is active with RNA polymerase alone, is inhibited by AsiA, and is activated by MotA/AsiA. We find that substitutions within the N-terminal half of AsiA, at amino acid residues V14, L18, and I40, rendered the protein defective for binding to sigma(70). These residues reside at the monomer-monomer interface in recent NMR structures of the AsiA dimer. In contrast, AsiA missing the C-terminal 44 amino acid residues interacted well with sigma(70) region 4 in the two-hybrid assay, and AsiA missing the C-terminal 17 amino acid residues (Delta74-90) bound to sigma(70) and was fully competent in standard in vitro transcription assays. However, the presence of the C-terminal region delayed formation of transcriptionally competent species when the AsiA/polymerase complex was pre-incubated with the promoter in the absence of MotA. Our results suggest that amino acid residues within the N-terminal half of AsiA are involved in forming or maintaining the AsiA/sigma(70) complex. The C-terminal region of AsiA, while not absolutely required for inhibition or co-activation, aids inhibition by slowing the formation of transcription complexes between a promoter and the AsiA/polymerase complex.

The bacteriophage T4 transcription activator MotA interacts with the far-C-terminal region of the sigma70 subunit of Escherichia coli RNA polymerase

Transcription from bacteriophage T4 middle promoters uses Escherichia coli RNA polymerase together with the T4 transcriptional activator MotA and the T4 coactivator AsiA. AsiA binds tightly within the C-terminal portion of the sigma70 subunit of RNA polymerase, while MotA binds to the 9-bp MotA box motif, which is centered at -30, and also interacts with sigma70. We show here that the N-terminal half of MotA (MotA(NTD)), which is thought to include the activation domain, interacts with the C-terminal region of sigma70 in an E. coli two-hybrid assay. Replacement of the C-terminal 17 residues of sigma70 with comparable sigma38 residues abolishes the interaction with MotA(NTD) in this assay, as does the introduction of the amino acid substitution R608C. Furthermore, in vitro transcription experiments indicate that a polymerase reconstituted with a sigma70 that lacks C-terminal amino acids 604 to 613 or 608 to 613 is defective for MotA-dependent activation. We also show that a proteolyzed fragment of MotA that contains the C-terminal half (MotA(CTD)) binds DNA with a K(D(app)) that is similar to that of full-length MotA. Our results support a model for MotA-dependent activation in which protein-protein contact between DNA-bound MotA and the far-C-terminal region of sigma70 helps to substitute functionally for an interaction between sigma70 and a promoter -35 element.

Solution structure and stability of the anti-sigma factor AsiA: implications for novel functions

Anti-sigma factors regulate prokaryotic gene expression through interactions with specific sigma factors. The bacteriophage T4 anti-sigma factor AsiA is a molecular switch that both inhibits transcription from bacterial promoters and phage early promoters and promotes transcription at phage middle promoters through its interaction with the primary sigma factor of Escherichia coli, sigma(70). AsiA is an all-helical, symmetric dimer in solution. The solution structure of the AsiA dimer reveals a novel helical fold for the protomer. Furthermore, the AsiA protomer, surprisingly, contains a helix-turn-helix DNA binding motif, predicting a potential new role for AsiA. The AsiA dimer interface includes a substantial hydrophobic component, and results of hydrogen/deuterium exchange studies suggest that the dimer interface is the most stable region of the AsiA dimer. In addition, the residues that form the dimer interface are those that are involved in binding to sigma(70). The results promote a model whereby the AsiA dimer maintains the active hydrophobic surfaces and delivers them to sigma(70), where an AsiA protomer is displaced from the dimer via the interaction of sigma(70) with the same residues in AsiA that constitute the dimer interface.

Flipping a genetic switch by subunit exchange

The bacteriophage T4 AsiA protein is a multifunctional protein that simultaneously acts as both a repressor and activator of gene expression during the phage life cycle. These dual roles with opposing transcriptional consequences are achieved by modification of the host RNA polymerase in which AsiA binds to conserved region 4 (SR4) of sigma(70), altering the pathway of promoter selection by the holoenzyme. The mechanism by which AsiA flips this genetic switch has now been revealed, in part, from the three-dimensional structure of AsiA and the elucidation of its interaction with SR4. The structure of AsiA is that of a novel homodimer in which each monomer is constructed as a seven-helix bundle arranged in four overlapping helix-loop-helix elements. Identification of the protein interfaces for both the AsiA homodimer and the AsiA-sigma(70) complex reveals that these interfaces are coincident. Thus, the AsiA interaction with sigma(70) necessitates that the AsiA homodimer dissociate to form an AsiA-SR4 heterodimer, exchanging one protein subunit for another to alter promoter choice by RNA polymerase.

Domain 1.1 of the sigma(70) subunit of Escherichia coli RNA polymerase modulates the formation of stable polymerase/promoter complexes

The sigma 70 (sigma(70)) subunit of Escherichia coli RNA polymerase specifies transcription from promoters that are responsible for basal gene expression during vegetative growth. When sigma(70) is present within polymerase holoenzyme, two of its domains, 2.4 and 4.2, interact with sequences within the -10 and -35 regions, respectively, of promoter DNA. However, in free sigma(70), DNA binding is prevented by domain 1.1, the N-terminal domain of the protein. Previous work has demonstrated that the presence of domain 1.1 is required for efficient transcription initiation at the lambda promoter P(R). To investigate whether this is a general property of domain 1.1, we have used five promoters to compare polymerases with and without domain 1.1 in in vitro transcription assays, and in assays assessing the formation and decay of stable, pretranscription complexes. We find that the absence of domain 1.1 does not render the polymerase defective at all of these promoters. Depending on the promoter, the absence of domain 1.1 can promote or inhibit transcription initiation by affecting the formation of stable pretranscription complexes. However, domain 1.1 does not affect the stability of these complexes once they are formed. For polymerases containing domain 1.1, the efficiency of stable complex formation correlates with how well the -10 and -35 regions of a promoter match the ideal sigma(70) recognition sequences. However, when domain 1.1 is absent, having this match becomes less important in determining how efficiently stable complexes are made. We suggest that domain 1.1 influences initiation by constraining polymerase to assess a promoter primarily by the fitness of its -10 and -35 regions to the canonical sequences.

Substitutions in bacteriophage T4 AsiA and Escherichia coli sigma(70) that suppress T4 motA activation mutations

Bacteriophage T4 middle-mode transcription requires two phage-encoded proteins, the MotA transcription factor and AsiA coactivator, along with Escherichia coli RNA polymerase holoenzyme containing the sigma(70) subunit. A motA positive control (pc) mutant, motA-pc1, was used to select for suppressor mutations that alter other proteins in the transcription complex. Separate genetic selections isolated two AsiA mutants (S22F and Q51E) and five sigma(70) mutants (Y571C, Y571H, D570N, L595P, and S604P). All seven suppressor mutants gave partial suppressor phenotypes in vivo as judged by plaque morphology and burst size measurements. The S22F mutant AsiA protein and glutathione S-transferase fusions of the five mutant sigma(70) proteins were purified. All of these mutant proteins allowed normal levels of in vitro transcription when tested with wild-type MotA protein, but they failed to suppress the mutant MotA-pc1 protein in the same assay. The sigma(70) substitutions affected the 4.2 region, which binds the -35 sequence of E. coli promoters. In the presence of E. coli RNA polymerase without T4 proteins, the L595P and S604P substitutions greatly decreased transcription from standard E. coli promoters. This defect could not be explained solely by a disruption in -35 recognition since similar results were obtained with extended -10 promoters. The generalized transcriptional defect of these two mutants correlated with a defect in binding to core RNA polymerase, as judged by immunoprecipitation analysis. The L595P mutant, which was the most defective for in vitro transcription, failed to support E. coli growth.

Study of the interaction between bacteriophage T4 asiA and Escherichia coli sigma(70), using the yeast two-hybrid system: neutralization of asiA toxicity to E. coli cells by coexpression of a truncated sigma(70) fragment

The interaction of T4 phage-encoded anti-sigma factor, asiA, and Escherichia coli sigma(70) was studied by using the yeast two-hybrid system. Truncation of sigma(70) to identify the minimum region involved in the interaction showed that the fragment containing amino acid residues proximal to the C terminus (residues 547 to 603) was sufficient for complexing to asiA. Studies also indicated that some of the truncated C-terminal fragments (residues 493 to 613) had higher affinity for asiA as judged by the increased beta-galactosidase activity. It is proposed that the observed higher affinity may be due to the unmasking of the binding region of asiA on the sigma protein. Advantage was taken of the increased affinity of truncated sigma(70) fragments to asiA in designing a coexpression system wherein the toxicity of asiA expression in E. coli could be neutralized and the complex of truncated sigma(70) and asiA could be expressed in large quantities and purified.

Binding of the bacteriophage T4 transcriptional activator, MotA, to T4 middle promoter DNA: evidence for both major and minor groove contacts

During infection, the bacteriophage T4 transcriptional activator MotA, the co-activator AsiA, and host RNA polymerase are needed to transcribe from T4 middle promoters. Middle promoters contain a -10 region recognized by the sigma(70)subunit of RNA polymerase and a MotA box centered at -30 that is bound by MotA. We have investigated how the loss or modification of base determinants within the MotA box sequence 5'TTTGCTTTA3' (positions -34 to -26 of a middle promoter) affects MotA function. Gel retardation assays with mutant MotA boxes are consistent with the idea that MotA uses minor groove contacts upstream and major groove contacts downstream of the center GC, and does not require any specific base feature at the C.G base-pair at position -30. In particular, the 5-methyl residue on the thymine residue at position -29, a major groove contact, contributes to MotA binding, while converting the T.A at -32 to a C. I base-pair, a change that affects the major but nor the minor groove, yields a MotA box that is similar to wild-type. However, methylation interference analyses indicate that neither the binding of MotA nor the binding of polymerase/MotA/AsiA to the middle promoter PuvsXis inhibited by premethylation of guanine and adenine residues, suggesting that binding does not require minor groove contact with any specific T.A base-pair. Using gel retardation analyses, we calculate an apparent dissociation constant of 130 nM for MotA binding to the wild-type MotA box. Previous work has shown that the N-terminal region of MotA is needed for an interaction between MotA and sigma(70). We suggest that this MotA-sigma(70)interaction helps to stabilize the relatively weak interaction of MotA with the -30 region of middle promoter DNA.

The bacteriophage T4 transcriptional activator MotA accepts various base-pair changes within its binding sequence

During infection, bacteriophage T4 regulates three sets of genes: early, middle, and late. The host RNA polymerase is capable of transcribing early genes, but middle transcription requires the T4-encoded transcriptional activator, MotA protein, and the T4 co-activator, AsiA protein, both of which bind to the sigma 70 (sigma70) subunit of RNA polymerase. MotA also binds a DNA sequence (a MotA box), centered at position -30. The identification of more than 20 middle promoters suggested that a strong match to the MotA box consensus sequence (t/a)(t/a)TGCTT(t/c)A was critical for MotA activation. We have investigated how specific base changes within the MotA box sequence affect MotA binding and activation in vitro, and we have identified seven new middle promoters in vivo. We find that an excellent match to the sigma70 -10 consensus sequence, rather than an excellent match to the MotA box consensus sequence, is an invariant feature of MotA-dependent promoters. Many single base changes in the MotA box are tolerated in binding and activation assays, indicating that there is more flexibility in the sequence requirements for MotA than was previously appreciated. We also find that using the natural T4 DNA, which contains glucosylated, 5-hydoxymethylated cytosine residues, affects the ability of particular MotA box sequences to activate transcription. We suggest that MotA and AsiA may function like certain eukaryotic TAFs (TATA binding protein (TBP) associated factors) whose binding to TBP results in transcription from new core promoter sequences.

The anti-sigma factors

A mechanism for regulating gene expression at the level of transcription utilizes an antagonist of the sigma transcription factor known as the anti-sigma (anti-sigma) factor. The cytoplasmic class of anti-sigma factors has been well characterized. The class includes AsiA form bacteriophage T4, which inhibits Escherichia coli sigma 70; FlgM, present in both gram-positive and gram-negative bacteria, which inhibits the flagella sigma factor sigma 28; SpoIIAB, which inhibits the sporulation-specific sigma factor, sigma F and sigma G, of Bacillus subtilis; RbsW of B. subtilis, which inhibits stress response sigma factor sigma B; and DnaK, a general regulator of the heat shock response, which in bacteria inhibits the heat shock sigma factor sigma 32. In addition to this class of well-characterized cytoplasmic anti-sigma factors, a new class of homologous, inner-membrane-bound anti-sigma factors has recently been discovered in a variety of eubacteria. This new class of anti-sigma factors regulates the expression of so-called extracytoplasmic functions, and hence is known as the ECF subfamily of anti-sigma factors. The range of cell processes regulated by anti-sigma factors is highly varied and includes bacteriophage phage growth, sporulation, stress response, flagellar biosynthesis, pigment production, ion transport, and virulence.

The bacteriophage T4 AsiA protein: a molecular switch for sigma 70-dependent promoters

The AsiA protein, encoded by bacteriophage T4, inhibits Esigma70-dependent transcription at bacterial and early-phage promoters. We demonstrate that the inhibitory action of AsiA involves interference with the recognition of the -35 consensus promoter sequence by host RNA polymerase. In vitro experiments were performed with a C-terminally labelled sigma factor that is competent for functional holoenzyme reconstitution. By protease and hydroxyl radical protein footprinting, we show that AsiA binds region 4.2 of sigma70, which recognizes the -35 sequence. Direct interference with the recognition of the promoter at this locus is supported by two parallel experiments. The stationary-phase sigma factor containing holoenzyme, which can initiate transcription at promoters devoid of a -35 region, is insensitive to AsiA inhibition. The recognition of a galP1 promoter by Esigma70 is not affected by the presence of AsiA. Therefore, we conclude that AsiA inhibits transcription from Escherichia coli and T4 early promoters by counteracting the recognition of region 4.2 of sigma70 with the -35 hexamer.

Interaction of bacteriophage T4 AsiA protein with Escherichia coli sigma70 and its variant

Bacteriophage T4 produces a small protein AsiA, which inhibits transcription from sigma70-dependent promoters in E. coli by tightly binding to sigma70 and is therefore termed as anti-sigma factor. We observed that there was no inhibition of single round transcription at lac UV5 promoter when AsiA was added to preformed open complex between RNA polymerase and template DNA. However, transcription was found to proceed normally at 'extended -10' promoters in the presence of AsiA. It appears therefore that AsiA binds sigma70 at its 4.2-subdomain or in its close vicinity. Further experiments on immunoprecipitation of sigma70 and a mutant sigma70-V576G with AsiA seem to corroborate such conclusion.

An N-terminal mutation in the bacteriophage T4 motA gene yields a protein that binds DNA but is defective for activation of transcription

The bacteriophage T4 MotA protein is a transcriptional activator of T4-modified host RNA polymerase and is required for activation of the middle class of T4 promoters. MotA alone binds to the -30 region of T4 middle promoters, a region that contains the MotA box consensus sequence [(t/a)(t/a)TGCTT(t/c)A]. We report the isolation and characterization of a protein designated Mot21, in which the first 8 codons of the wild-type motA sequence have been replaced with 11 different codons. In gel retardation assays, Mot21 and MotA bind DNA containing the T4 middle promoter P(uvsX) similarly, and the proteins yield similar footprints on P(uvsX). However, Mot21 is severely defective in the activation of transcription. On native protein gels, a new protein species is seen after incubation of the sigma70 subunit of RNA polymerase and wild-type MotA protein, suggesting a direct protein-protein contact between MotA and sigma70. Mot21 fails to form this complex, suggesting that this interaction is necessary for transcriptional activation and that the Mot21 defect arises because Mot21 cannot form this contact like the wild-type activator.