Interactions of the TnaC nascent peptide with rRNA in the exit tunnel enable the ribosome to respond to free tryptophan

A transcriptional attenuation mechanism regulates expression of the bacterial tnaCAB operon. This mechanism requires ribosomal arrest induced by the regulatory nascent TnaC peptide in response to free L-tryptophan (L-Trp). In this study we demonstrate, using genetic and biochemical analyses, that in Escherichia coli, TnaC residue I19 and 23S rRNA nucleotide A2058 are essential for the ribosome's ability to sense free L-Trp. We show that the mutational change A2058U in 23S rRNA reduces the concentration dependence of L-Trp-mediated tna operon induction, whereas the TnaC I19L change suppresses this phenotype, restoring the sensitivity of the translating A2058U mutant ribosome to free L-Trp. These findings suggest that interactions between TnaC residue I19 and 23S rRNA nucleotide A2058 contribute to the creation of a regulatory L-Trp binding site within the translating ribosome.

Isolation of translating ribosomes containing peptidyl-tRNAs for functional and structural analyses

Recently, structural and biochemical studies have detailed many of the molecular events that occur in the ribosome during inhibition of protein synthesis by antibiotics and during nascent polypeptide synthesis. Some of these antibiotics, and regulatory nascent polypeptides mostly in the form of peptidyl-tRNAs, inhibit either peptide bond formation or translation termination^1-7. These inhibitory events can stop the movement of the ribosome, a phenomenon termed "translational arrest". Translation arrest induced by either an antibiotic or a nascent polypeptide has been shown to regulate the expression of genes involved in diverse cellular functions such as cell growth, antibiotic resistance, protein translocation and cell metabolism^8-13. Knowledge of how antibiotics and regulatory nascent polypeptides alter ribosome function is essential if we are to understand the complete role of the ribosome in translation, in every organism.

Here, we describe a simple methodology that can be used to purify, exclusively, for analysis, those ribosomes translating a specific mRNA and containing a specific peptidyl-tRNA¹⁴. This procedure is based on selective isolation of translating ribosomes bound to a biotin-labeled mRNA. These translational complexes are separated from other ribosomes in the same mixture, using streptavidin paramagnetic beads (SMB) and a magnetic field (MF). Biotin-labeled mRNAs are synthesized by run-off transcription assays using as templates PCR-generated DNA fragments that contain T7 transcriptional promoters. T7 RNA polymerase incorporates biotin-16-UMP from biotin-UTP; under our conditions approximately ten biotin-16-UMP molecules are incorporated in a 600 nt mRNA with a 25% UMP content. These biotin-labeled mRNAs are then isolated, and used in in vitro translation assays performed with release factor 2 (RF2)-depleted cell-free extracts obtained from Escherichia coli strains containing wild type or mutant ribosomes. Ribosomes translating the biotin-labeled mRNA sequences are stalled at the stop codon region, due to the absence of the RF2 protein, which normally accomplishes translation termination. Stalled ribosomes containing the newly synthesized peptidyl-tRNA are isolated and removed from the translation reactions using SMB and an MF. These beads only bind biotin-containing messages.

The isolated, translational complexes, can be used to analyze the structural and functional features of wild type or mutant ribosomal components, or peptidyl-tRNA sequences, as well as determining ribosome interaction with antibiotics or other molecular factors ^1,14-16. To examine the function of these isolated ribosome complexes, peptidyl-transferase assays can be performed in the presence of the antibiotic puromycin¹. To study structural changes in translational complexes, well established procedures can be used, such as i) crosslinking to specific amino acids¹⁴ and/or ii) alkylation protection assays^1,14,17.

Efficient cloning of genes of Neurospora crassa

We have constructed a genomic library of Neurospora crassa DNA in a cosmid vector that contains the dominant selectable marker for benomyl resistance. The library is arranged to permit the rapid cloning of Neurospora genes by either sib-selection or colony-hybridization protocols. Detailed procedures for the uses of the library are described. By use of these procedures, a modest number of unrelated genes have been isolated. The cloning of trp-3, the structural gene for the multifunctional enzyme tryptophan synthetase (tryptophan synthase, EC 4.2.1.20), is reported in detail; its identity was verified by restriction fragment length polymorphism mapping. The strategies described in this paper should be of use in the cloning of any gene of Neurospora, as well as genes of other lower eukaryotes.

Evolution of bacterial trp operons and their regulation

Survival and replication of most bacteria require the ability to synthesize the amino acid L-tryptophan whenever it is not available from the environment. In this article we describe the genes, operons, proteins, and reactions involved in tryptophan biosynthesis in bacteria, and the mechanisms they use in regulating tryptophan formation. We show that although the reactions of tryptophan biosynthesis are essentially identical, gene organization varies among species--from whole-pathway operons to completely dispersed genes. We also show that the regulatory mechanisms used for these genes vary greatly. We address the question--what are some potential advantages of the gene organization and regulation variation associated with this conserved, important pathway?

RNA-based regulation of genes of tryptophan synthesis and degradation, in bacteria

We are now aware that RNA-based regulatory mechanisms are commonly used to control gene expression in many organisms. These mechanisms offer the opportunity to exploit relatively short, unique RNA sequences, in altering transcription, translation, and/or mRNA stability, in response to the presence of a small or large signal molecule. The ability of an RNA segment to fold and form alternative hairpin secondary structures -- each dedicated to a different regulatory function -- permits selection of specific sequences that can affect transcription and/or translation. In the present paper I will focus on our current understanding of the RNA-based regulatory mechanisms used by Escherichia coli and Bacillus subtilis in controlling expression of the tryptophan biosynthetic operon. The regulatory mechanisms they use for this purpose differ, suggesting that these organisms, or their ancestors, adopted different strategies during their evolution. I will also describe the RNA-based mechanism used by E. coli in regulating expression of its operon responsible for tryptophan degradation, the tryptophanase operon.

Comparison of tryptophan biosynthetic operon regulation in different Gram-positive bacterial species

The tryptophan biosynthetic operon has been widely used as a model system for studying transcription regulation. In Bacillus subtilis, the trp operon is primarily regulated by a tryptophan-activated RNA-binding protein, TRAP. Here we show that in many other Gram-positive species the trp operon is regulated differently, by tRNA(Trp) sensing by the RNA-based T-box mechanism, with T-boxes arranged in tandem. Our analyses reveal an apparent relationship between trp operon organization and the specific regulatory mechanism(s) used.

Establishing the Triplet Nature of the Genetic Code

In 1961, Crick, Barnett, Brenner, and Watts-Tobin (Crick et al., 1961) designed an elegant experimental strategy to determine the nature of the genetic code. Remarkably, they reached the correct conclusion despite the absence of technology to analyze and compare DNA and protein sequences.

Ribosome recycling factor and release factor 3 action promotes TnaC-peptidyl-tRNA Dropoff and relieves ribosome stalling during tryptophan induction of tna operon expression in Escherichia coli

Upon tryptophan induction of tna operon expression in Escherichia coli, the leader peptidyl-tRNA, TnaC-tRNA(2)(Pro), resists cleavage, resulting in ribosome stalling at the tnaC stop codon. This stalled ribosome blocks Rho factor binding and action, preventing transcription termination in the tna operon's leader region. Plasmid-mediated overexpression of tnaC was previously shown to inhibit cell growth by reducing uncharged tRNA(2)(Pro) availability. Which factors relieve ribosome stalling, facilitate TnaC-tRNA(2)(Pro) cleavage, and relieve growth inhibition were addressed in the current study. In strains containing the chromosomal tna operon and lacking a tnaC plasmid, the overproduction of ribosome recycling factor (RRF) and release factor 3 (RF3) reduced tna operon expression. Their overproduction in vivo also increased the rate of cleavage of TnaC-tRNA(2)(Pro), relieving the growth inhibition associated with plasmid-mediated tnaC overexpression. The overproduction of elongation factor G or initiation factor 3 did not have comparable effects, and tmRNA was incapable of attacking TnaC-tRNA(2)(Pro) in stalled ribosome complexes. The stability of TnaC-tRNA(2)(Pro) was increased appreciably in strains deficient in RRF and RF3 or deficient in peptidyl-tRNA hydrolase. These findings reveal the existence of a natural mechanism whereby an amino acid, tryptophan, binds to ribosomes that have just completed the synthesis of TnaC-tRNA(2)(Pro). Bound tryptophan inhibits RF2-mediated cleavage of TnaC-tRNA(2)(Pro), resulting in the stalling of the ribosome translating tnaC mRNA. This stalling results in increased transcription of the structural genes of the tna operon. RRF and RF3 then bind to this stalled ribosome complex and slowly release TnaC-tRNA(2)(Pro). This release allows ribosome recycling and permits the cleavage of TnaC-tRNA(2)(Pro) by peptidyl-tRNA hydrolase.

Ribosomal features essential for tna operon induction: tryptophan binding at the peptidyl transferase center

Features of the amino acid sequence of the TnaC nascent peptide are recognized by the translating ribosome. Recognition leads to tryptophan binding within the translating ribosome, inhibiting the termination of tnaC translation and preventing Rho-dependent transcription termination in the tna operon leader region. It was previously shown that inserting an adenine residue at position 751 or introducing the U2609C change in 23S rRNA or introducing the K90W replacement in ribosomal protein L22 prevented tryptophan induction of tna operon expression. It was also observed that an adenine at position 752 of 23S rRNA was required for induction. In the current study, the explanation for the lack of induction by these altered ribosomes was investigated. Using isolated TnaC-ribosome complexes, it was shown that although tryptophan inhibits puromycin cleavage of TnaC-tRNA(Pro) with wild-type ribosome complexes, it does not inhibit cleavage with the four mutant ribosome complexes examined. Similarly, tryptophan prevents sparsomycin inhibition of TnaC-tRNA(Pro) cleavage with wild-type ribosome complexes but not with these mutant ribosome complexes. Additionally, a nucleotide located close to the peptidyl transferase center, A2572, which was protected from methylation by tryptophan with wild-type ribosome complexes, was not protected with mutant ribosome complexes. These findings identify specific ribosomal residues located in the ribosome exit tunnel that recognize features of the TnaC peptide. This recognition creates a free tryptophan-binding site in the peptidyl transferase center, where bound tryptophan inhibits peptidyl transferase activity.

A novel approach to speed up peptide sequencing via MS/MS spectra analysis

In proteomics, tandem mass spectrometry is the key technology for peptide sequencing from the cells. Different methods have been proposed to sequence peptides through tandem mass spectra. While the methods are capable of providing more robust and accurate results, they are also computationally expensive, and create a bottleneck in high throughput peptide identification. In this work, we introduce a novel approach to speedup peptide sequencing. In contrast to the traditional approaches, we conduct coarse comparison of spectral profiles to drastically shrink the size of candidate peptides. A fast algorithm has been developed for this goal. It is shown in our experiments that such an approach can significantly improve the speed for peptide sequencing.

Surface analysis of peptide mass spectra to improve time and mass localization

Current peak detections algorithms for processing mass spectrometry (MS) spectra generally rely on two dimensional techniques for identifying the location and intensity of peaks from a single spectrum. However, when high performance liquid chromatography (HPLC) is coupled with mass spectrometry, a third dimension, retention time, is introduced. The ensemble of MS spectra may then be regarded as a 3D surface where spectral intensity is a function of m/z (mass-to-charge) and time. This suggests that peak localization can be improved by incorporating the time domain data and average data across both dimensions. This work describes a surface intensity analysis algorithm and the results of its use.

Overexpression of tnaC of Escherichia coli inhibits growth by depleting tRNA2Pro availability

Transcription of the tryptophanase (tna) operon of Escherichia coli is regulated by catabolite repression and tryptophan-induced transcription antitermination. Induction results from ribosome stalling after translation of tnaC, the coding region for a 24-residue leader peptide. The last sense codon of tnaC, proline codon 24 (CCU), is translated by tRNA(2)(Pro). We analyzed the consequences of overexpression of tnaC from a multicopy plasmid and observed that under inducing conditions more than 60% of the tRNA(2)(Pro) in the cell was sequestered in ribosomes as TnaC-tRNA(2)(Pro). The half-life of this TnaC-tRNA(2)(Pro) was shown to be 10 to 15 min under these conditions. Plasmid-mediated overexpression of tnaC, under inducing conditions, reduced cell growth rate appreciably. Increasing the tRNA(2)(Pro) level relieved this growth inhibition, suggesting that depletion of this tRNA was primarily responsible for the growth rate reduction. Growth inhibition was not relieved by overexpression of tRNA(1)(Pro), a tRNA(Pro) that translates CCG, but not CCU. Replacing the Pro24CCU codon of tnaC by Pro24CCG, a Pro codon translated by tRNA(1)(Pro), also led to growth rate reduction, and this reduction was relieved by overexpression of tRNA(1)(Pro). These findings establish that the growth inhibition caused by tnaC overexpression during induction by tryptophan is primarily a consequence of tRNA(Pro) depletion, resulting from TnaC-tRNA(Pro) retention within stalled, translating ribosomes.

Changes produced by bound tryptophan in the ribosome peptidyl transferase center in response to TnaC, a nascent leader peptide

Studies in vitro have established that free tryptophan induces tna operon expression by binding to the ribosome that has just completed synthesis of TnaC-tRNA(Pro), the peptidyl-tRNA precursor of the leader peptide of this operon. Tryptophan acts by inhibiting Release Factor 2-mediated cleavage of this peptidyl-tRNA at the tnaC stop codon. Here we analyze the ribosomal location of free tryptophan, the changes it produces in the ribosome, and the role of the nascent TnaC-tRNA(Pro) peptide in facilitating tryptophan binding and induction. The positional changes of 23S rRNA nucleotides that occur during induction were detected by using methylation protection and binding/competition assays. The ribosome-TnaC-tRNA(Pro) complexes analyzed were formed in vitro; they contained either wild-type TnaC-tRNA(Pro) or its nonfunctional substitute, TnaC(W12R)-tRNA(Pro). Upon comparing these two peptidyl-tRNA-ribosome complexes, free tryptophan was found to block methylation of nucleotide A2572 of wild-type ribosome-TnaC-tRNA(Pro) complexes but not of ribosome-TnaC(W12R)-tRNA(Pro) complexes. Nucleotide A2572 is in the ribosomal peptidyl transferase center. Tryptophanol, a noninducing competitor of tryptophan, was ineffective in blocking A2572 methylation; however, it did reverse the protective effect of tryptophan. Free tryptophan inhibited puromycin cleavage of TnaC-tRNA(Pro); it also inhibited binding of the antibiotic sparsomycin. These effects were not observed with TnaC(W12R)-tRNA(Pro) mutant complexes. These findings establish that Trp-12 of TnaC-tRNA(Pro) is required for introducing specific changes in the peptidyl transferase center of the ribosome that activate free tryptophan binding, resulting in peptidyl transferase inhibition. Free tryptophan appears to act at or near the binding sites of several antibiotics in the peptidyl transferase center.

Complexity in regulation of tryptophan biosynthesis in Bacillus subtilis

Bacillus subtilis uses novel regulatory mechanisms in controlling expression of its genes of tryptophan synthesis and transport. These mechanisms respond to changes in the intracellular concentrations of free tryptophan and uncharged tRNA(Trp). The major B. subtilis protein that regulates tryptophan biosynthesis is the tryptophan-activated RNA-binding attenuation protein, TRAP. TRAP is a ring-shaped molecule composed of 11 identical subunits. Active TRAP binds to unique RNA segments containing multiple trinucleotide (NAG) repeats. Binding regulates both transcription termination and translation in the trp operon, and translation of other coding regions relevant to tryptophan metabolism. When there is a deficiency of charged tRNA(Trp), B. subtilis forms an anti-TRAP protein, AT. AT antagonizes TRAP function, thereby increasing expression of all the genes regulated by TRAP. Thus B. subtilis and Escherichia coli respond to identical regulatory signals, tryptophan and uncharged tRNA(Trp), yet they employ different mechanisms in regulating trp gene expression.

Features of ribosome-peptidyl-tRNA interactions essential for tryptophan induction of tna operon expression

Certain nascent peptide sequences, when within the ribosomal exit tunnel, can inhibit translation termination and/or peptide elongation. The 24 residue leader peptidyl-tRNA of the tna operon of E. coli, TnaC-tRNA(Pro), in the presence of excess tryptophan, resists cleavage at the tnaC stop codon. TnaC residue Trp12 is crucial for this inhibition. The approximate location of Trp12 in the exit tunnel was determined by crosslinking Lys11 of TnaC-tRNA(Pro) to nucleotide A750 of 23S rRNA. Methylation of nucleotide A788 of 23S rRNA was reduced by the presence of Trp12 in TnaC-tRNA(Pro), implying A788 displacement. Inserting an adenylate at position 751, or introducing the change U2609C in 23S rRNA or the change K90H or K90W in ribosomal protein L22, virtually eliminated tryptophan induction. These modified and mutated regions are mostly located near the putative site occupied by Trp12 of TnaC-tRNA(Pro). These findings identify features of the ribosomal exit tunnel essential for tna operon induction.

New insights into regulation of the tryptophan biosynthetic operon in Gram-positive bacteria

The tryptophan operon of Bacillus subtilis serves as an excellent model for investigating transcription regulation in Gram-positive bacteria. In this article, we extend this knowledge by analyzing the predicted regulatory regions in the trp operons of other fully sequenced Gram-positive bacteria. Interestingly, it appears that in eight of the organisms examined, transcription of the trp operon appears to be regulated by tandem T-box elements. These regulatory elements have recently been described in the trp operons of two bacterial species. Single T-box elements are commonly found in Gram-positive bacteria in operons encoding aminoacyl tRNA synthetases and proteins performing other functions. Different regulatory mechanisms appear to be associated with variations of trp gene organization within the trp operon.

Transcription attenuation: a highly conserved regulatory strategy used by bacteria

We employed computational analyses to assess the conservation of sequence elements that are believed to be essential for the various transcription-attenuation (termination) mechanisms that are used to regulate expression of families of orthologous genes in bacteria. We searched the upstream sequence of every predicted transcription unit for a transcription attenuator. These were then clustered by the orthology relationships of the nearby structural genes. Many gene families regulated by transcription attenuation were found to be adjacent to a regulatory region that had a binding site for a specific protein, tRNA or small metabolite. Using our methodology, we predict that at least 80 different clusters of orthologous groups (COGs) are significantly regulated by transcription attenuation.

Effects of tryptophan starvation on levels of the trp RNA-binding attenuation protein (TRAP) and anti-TRAP regulatory protein and their influence on trp operon expression in Bacillus subtilis

The anti-TRAP protein (AT), encoded by the rtpA gene of Bacillus subtilis, can bind to and inhibit the tryptophan-activated trp RNA-binding attenuation protein (TRAP). AT binding can prevent TRAP from promoting transcription termination in the leader region of the trp operon, thereby increasing trp operon expression. We show here that AT levels continue to increase as tryptophan starvation becomes more severe, whereas the TRAP level remains relatively constant and independent of tryptophan starvation. Assuming that the functional form of AT is a trimer, we estimate that the ratios of AT trimers per TRAP molecule are 0.39 when the cells are grown under mild tryptophan starvation conditions, 0.83 under more severe starvation conditions, and approximately 2.0 when AT is expressed maximally. As the AT level is increased, a corresponding increase is observed in the anthranilate synthase level. When AT is expressed maximally, the anthranilate synthase level is about 70% of the level observed in a strain lacking TRAP. In a nutritional shift experiment where excess phenylalanine and tyrosine could potentially starve cells of tryptophan, both the AT level and anthranilate synthase activity were observed to increase. Expression of the trp operon is clearly influenced by the level of AT.

The different roles of tryptophan transfer RNA in regulating trp operon expression in E. coli versus B. subtilis

Escherichia coli and Bacillus subtilis use different mechanisms of sensing and responding to tryptophan and uncharged tRNA(Trp) as regulatory signals. In E. coli, tryptophan activates a repressor that binds to the trp promoter- operator, inhibiting transcription initiation. In B. subtilis, tryptophan activates an RNA-binding protein, TRAP, which binds to the trp operon leader RNA, causing transcription termination. In E. coli uncharged tRNA(Trp) accumulation stalls the ribosome attempting translation of tandem Trp codons in the leader-peptide coding region of the operon. This stalling permits the formation of an RNA antiterminator structure, preventing transcription termination. In B. subtilis uncharged tRNA(Trp) accumulation activates transcription and translation of the at operon. AT protein inhibits tryptophan-activated TRAP, thereby preventing TRAP-mediated transcription termination. These differences might reflect the unique organizational features of the respective trp operons and their ancestry.

Features of a leader peptide coding region that regulate translation initiation for the anti-TRAP protein of B. subtilis

The rtpA gene of Bacillus subtilis encodes the Anti-TRAP protein, AT. AT can bind and inhibit the TRAP regulatory protein, preventing TRAP from promoting transcription termination in the trpEDCFBA operon leader region. AT synthesis is upregulated transcriptionally and translationally in response to the accumulation of uncharged tRNA(Trp). Here we analyze AT's translational regulation by rtpLP, a 10 residue leader peptide coding region located immediately preceding the rtpA Shine-Dalgarno sequence. Our findings suggest that, whenever the charged tRNA(Trp) level is sufficient to allow the ribosome translating rtpLP to reach its stop codon, it blocks the adjacent rtpA Shine-Dalgarno sequence, inhibiting AT synthesis. However, when there is a charged tRNA(Trp) deficiency, the translating ribosome presumably stalls at one of three adjacent rtpLP Trp codons. This stalling exposes the rtpA Shine-Dalgarno sequence, permitting AT synthesis. RNA-RNA pairing may also influence AT synthesis. Production of AT would inactivate TRAP, thereby increasing trp operon expression.

A transcriptional pause synchronizes translation with transcription in the tryptophanase operon leader region

Regulation of transcription of the tryptophanase operon requires that translation of its leader peptide coding region, tnaC, be coupled with its transcription. We show in vitro that a transcription pause site exists at the end of the tnaC coding region and that translation of tnaC releases the paused transcription complex, coupling transcription with translation.

Tandem transcription and translation regulatory sensing of uncharged tryptophan tRNA

The Bacillus subtilis AT (anti-TRAP) protein inhibits the regulatory protein TRAP (trp RNA-binding attenuation protein), thereby eliminating transcription termination in the leader region of the trp operon. Transcription of the AT operon is activated by uncharged tryptophan transfer RNA (tRNATrp). Here we show that translation of AT also is regulated by uncharged tRNATrp. A 10-residue coding region containing three consecutive tryptophan codons is located immediately preceding the AT structural gene. Completion of translation of this coding region inhibits AT synthesis, whereas incomplete translation increases AT production. Tandem sensing of uncharged tRNATrp therefore regulates synthesis of AT, which in turn regulates TRAP's ability to inhibit trp operon expression.

Genomewide transcriptional changes associated with genetic alterations and nutritional supplementation affecting tryptophan metabolism in Bacillus subtilis

DNA microarrays comprising approximately 95% of the Bacillus subtilis annotated protein coding ORFs were deployed to generate a series of snapshots of genomewide transcriptional changes that occur when cells are grown under various conditions that are expected to increase or decrease transcription of the trp operon segment of the aromatic supraoperon. Comparisons of global expression patterns were made between cells grown in the presence of indole acrylic acid, a specific inhibitor of tRNA(Trp) charging; cells deficient in expression of the mtrB gene, which encodes the tryptophan-activated negative regulatory protein, TRAP; WT cells grown in the presence or absence of two or three of the aromatic amino acids; and cells harboring a tryptophanyl tRNA synthetase mutation conferring temperature-sensitive tryptophan-dependent growth. Our findings validate expected responses of the tryptophan biosynthetic genes and presumed regulatory interrelationships between genes in the different aromatic amino acid pathways and the histidine biosynthetic pathway. Using a combination of supervised and unsupervised statistical methods we identified approximately 100 genes whose expression profiles were closely correlated with those of the genes in the trp operon. This finding suggests that expression of these genes is influenced directly or indirectly by regulatory events that affect or are a consequence of altered tryptophan metabolism.

Zinc is required for assembly and function of the anti-trp RNA-binding attenuation protein, AT

The anti-TRAP protein (AT) of Bacillus subtilis regulates expression of the trp operon and other genes concerned with tryptophan metabolism. AT acts by inhibiting the tryptophan-activated trp RNA-binding attenuation protein (TRAP). AT is an oligomer of identical 53-residue polypeptides; it is produced in response to the accumulation of uncharged tRNA(Trp). Each AT polypeptide has two cysteine-rich clusters that correspond to the signature motif of the cysteine-rich zinc-binding domain of the chaperone protein DnaJ. Here we characterize the putative zinc-binding domain of AT and establish the importance of zinc for AT assembly and activity. AT is shown to contain Zn(II) at a ratio of one ion per monomer. Bound zinc is necessary for maintenance of the quaternary structure of AT; the removal of zinc converts the AT complex into inactive monomers. All four cysteine residues in the AT polypeptide are involved in Zn(II) coordination. Chemical cross-linking analyses indicate that the AT functional oligomer is a hexamer composed of two trimers. Substituting alanine for any cysteine residue of AT results in rapid degradation of the mutant protein in vivo. We propose a model for the AT trimer in which three AT chains are held together by three zinc atoms, each coordinated by the N-terminal segment and the C-terminal segment of separate AT polypeptides.

Instruction of translating ribosome by nascent peptide

Expression of the tryptophanase operon of Escherichia coli is regulated by catabolite repression and tryptophan-induced transcription antitermination. An induction site activated by l-tryptophan is created in the translating ribosome during synthesis of TnaC, the 24-residue leader peptide. Replacing the tnaC stop codon with a tryptophan codon allows tryptophan-charged tryptophan transfer RNA to substitute for tryptophan as inducer. This suggests that the ribosomal A site occupied by the tryptophanyl moiety of the charged transfer RNA is the site of induction. The location of tryptophan-12 of nascent TnaC in the peptide exit tunnel was crucial for induction. These results show that a nascent peptide sequence can influence translation continuation and termination within a translating ribosome.

Regulation by transcription attenuation in bacteria: how RNA provides instructions for transcription termination/antitermination decisions

Regulation of gene expression by premature termination of transcription, or transcription attenuation, is a common regulatory strategy in bacteria. Various mechanisms of regulating transcription termination have been uncovered, each can be placed in either of two broad categories of termination events. Many mechanisms involve choosing between two alternative hairpin structures in an RNA transcript, with the decision dependent on interactions between ribosome and transcript, tRNA and transcript, or protein and transcript. In other examples, modification of the transcription elongation complex is the crucial event. This article will describe and compare several of these regulatory strategies, and will cite specific examples to illustrate the different mechanisms employed.

Analysis of tryptophanase operon expression in vitro: accumulation of TnaC-peptidyl-tRNA in a release factor 2-depleted S-30 extract prevents Rho factor action, simulating induction

Expression of the tryptophanase (tna) operon in Escherichia coli is regulated by catabolite repression and tryptophan-induced transcription antitermination. The key feature of this antitermination mechanism has been shown to be the retention of uncleaved TnaC-peptidyl-tRNA in the translating ribosome. This ribosome remains stalled at the tna stop codon and blocks the access of Rho factor to the tna transcript, thereby preventing transcription termination. In normal S-30 preparations, synthesis of a TnaC peptide containing arginine instead of tryptophan at position 12 (Arg(12)-TnaC) was shown to be insensitive to added tryptophan, i.e. Arg(12)-TnaC-peptidyl-tRNA was cleaved, and there was normal Rho-dependent transcription termination. When the S-30 extract used was depleted of release factor 2, Arg(12)-TnaC-tRNA(Pro) was accumulated in the absence or presence of added tryptophan. Under these conditions the accumulation of Arg(12)-TnaC-tRNA(Pro) prevented Rho-dependent transcription termination, mimicking normal induction. Using a minimal in vitro transcription system consisting of a tna template, RNA polymerase, and Rho, it was shown that RNA sequences immediately adjacent to the tnaC stop codon, the presumed boxA and rut sites, contributed most significantly to Rho-dependent termination. The tna boxA-like sequence appeared to serve as a segment of the Rho "entry" site, despite its likeness to the boxA element.

The anti-trp RNA-binding attenuation protein (Anti-TRAP), AT, recognizes the tryptophan-activated RNA binding domain of the TRAP regulatory protein

In Bacillus subtilis, the trp RNA-binding attenuation protein (TRAP) regulates expression of genes involved in tryptophan metabolism in response to the accumulation of l-tryptophan. Tryptophan-activated TRAP negatively regulates expression by binding to specific mRNA sequences and either promoting transcription termination or blocking translation initiation. Conversely, the accumulation of uncharged tRNA(Trp) induces synthesis of an anti-TRAP protein (AT), which forms a complex with TRAP and inhibits its activity. In this report, we investigate the structural features of TRAP required for AT recognition. A collection of TRAP mutant proteins was examined that were known to be partially or completely defective in tryptophan binding and/or RNA binding. Analyses of AT interactions with these proteins were performed using in vitro transcription termination assays and cross-linking experiments. We observed that TRAP mutant proteins that had lost the ability to bind RNA were no longer recognized by AT. Our findings suggest that AT acts by competing with messenger RNA for the RNA binding domain of TRAP. B. subtilis AT was also shown to interact with TRAP proteins from Bacillus halodurans and Bacillus stearothermophilus, implying that the structural elements required for AT recognition are conserved in the TRAP proteins of these species. Analyses of AT interaction with B. stearothermophilus TRAP at 60 degrees C demonstrated that AT is active at this elevated temperature.

Advancing our knowledge in biochemistry, genetics, and microbiology through studies on tryptophan metabolism

I was fortunate to practice science during the last half of the previous century, when many basic biological and biochemical concepts could be experimentally addressed for the first time. My introduction to research involved isolating and identifying intermediates in the niacin biosynthetic pathway. These studies were followed by investigations focused on determining the properties of genes and enzymes essential to metabolism and examining how they were alterable by mutation. The most challenging problem I initially attacked was establishing the colinear relationship between gene and protein. Subsequent research emphasized identification and characterization of regulatory mechanisms that microorganisms use to control gene expression. An elaborate regulatory strategy, transcription attenuation, was discovered that is often based on selection between alternative RNA structures. Throughout my career I enjoyed the excitement of solving basic scientific problems. Most rewarding, however, was the feeling that I was helping young scientists experience the pleasure of performing creative research.

Inhibition of the B. subtilis regulatory protein TRAP by the TRAP-inhibitory protein, AT

An anti-TRAP (AT) protein, a factor of previously unknown function, conveys the metabolic signal that the cellular transfer RNA for tryptophan (tRNATrp) is predominantly uncharged. Expression of the operon encoding AT is induced by uncharged tRNATrp. AT associates with TRAP, the trp operon attenuation protein, and inhibits its binding to its target RNA sequences. This relieves TRAP-mediated transcription termination and translation inhibition, increasing the rate of tryptophan biosynthesis. AT binds to TRAP primarily when it is in the tryptophan-activated state. The 53-residue AT polypeptide is homologous to the zinc-binding domain of DnaJ. The mechanisms regulating tryptophan biosynthesis in Bacillus subtilis differ from those used by Escherichia coli.

The mechanism of tryptophan induction of tryptophanase operon expression: tryptophan inhibits release factor-mediated cleavage of TnaC-peptidyl-tRNA(Pro)

Expression of the tryptophanase (tna) operon of Escherichia coli is regulated by catabolite repression and tryptophan-induced transcription antitermination. In a previous study, we reproduced the regulatory features of this operon observed in vivo by using an in vitro S-30 system. We also found that, under inducing conditions, the leader peptidyl-tRNA (TnaC-peptidyl-tRNA(Pro)) is not cleaved; it accumulates in the S-30 reaction mixture. In this paper, we examine the requirements for TnaC-peptidyl-tRNA(Pro) accumulation and cleavage, in vitro. We show that this peptidyl-tRNA remains bound to the translating ribosome. Removal of free tryptophan and addition of release factor 1 or 2 leads to hydrolysis of TnaC-peptidyl-tRNA(Pro) and release of TnaC from the ribosome-mRNA complex. Release factor-mediated cleavage is prevented by the addition of tryptophan. TnaC of the ribosome-bound TnaC-peptidyl-tRNA(Pro) was transferable to puromycin. This transfer was also blocked by tryptophan. Tests with various tryptophan analogs as substitutes for tryptophan revealed the existence of strict structural requirements for tryptophan action. Our findings demonstrate that the addition of tryptophan to ribosomes bearing nascent TnaC-peptidyl-tRNA(Pro) inhibits both TnaC peptidyl-tRNA(Pro) hydrolysis and TnaC peptidyl transfer. The associated translating ribosome therefore remains attached to the leader transcript where it blocks Rho factor binding and subsequent transcription termination.

The structures of anthranilate synthase of Serratia marcescens crystallized in the presence of (i) its substrates, chorismate and glutamine, and a product, glutamate, and (ii) its end-product inhibitor, L-tryptophan

The crystal structure of anthranilate synthase (AS) from Serratia marcescens, a mesophilic bacterium, has been solved in the presence of its substrates, chorismate and glutamine, and one product, glutamate, at 1.95 A, and with its bound feedback inhibitor, tryptophan, at 2.4 A. In comparison with the AS structure from the hyperthermophile Sulfolobus solfataricus, the S. marcescens structure shows similar subunit structures but a markedly different oligomeric organization. One crystal form of the S. marcescens enzyme displays a bound pyruvate as well as a putative anthranilate (the nitrogen group is ambiguous) in the TrpE subunit. It also confirms the presence of a covalently bound glutamyl thioester intermediate in the TrpG subunit. The tryptophan-bound form reveals that the inhibitor binds at a site distinct from that of the substrate, chorismate. Bound tryptophan appears to prevent chorismate binding by a demonstrable conformational effect, and the structure reveals how occupancy of only one of the two feedback inhibition sites can immobilize the catalytic activity of both TrpE subunits. The presence of effectors in the structure provides a view of the locations of some of the amino acid residues in the active sites. Our findings are discussed in terms of the previously described AS structure of S. solfataricus, mutational data obtained from enteric bacteria, and the enzyme's mechanism of action.

Reproducing tna operon regulation in vitro in an S-30 system. Tryptophan induction inhibits cleavage of TnaC peptidyl-tRNA

Expression of the tryptophanase (tna) operon of Escherichia coli is regulated by catabolite repression and tryptophan-induced transcription antitermination. Catabolite repression regulates transcription initiation, whereas excess tryptophan induces antitermination at Rho factor-dependent termination sites in the leader region of the operon. Synthesis of the leader peptide, TnaC, is essential for antitermination. BoxA and rut sites in the immediate vicinity of the tnaC stop codon are required for termination. In this paper we use an in vitro S-30 cell-free system to analyze the features of tna operon regulation. We show that transcription initiation is cyclic AMP (cAMP)-dependent and is not influenced by tryptophan. Continuation of transcription beyond the leader region requires the presence of inducing levels of tryptophan and synthesis of the TnaC leader peptide. Using a tnaA'-'trpE fusion, we demonstrate that induction results in a 15-20-fold increase in synthesis of the tryptophan-free TnaA-TrpE fusion protein. Replacing Trp codon 12 of tnaC by an Arg codon, or changing the tnaC start codon to a stop codon, eliminates induction. Addition of bicyclomycin, a specific inhibitor of Rho factor action, substantially increases basal level expression. Analyses of tna mRNA synthesis in vitro demonstrate that, in the absence of inducer transcription is terminated and the terminated transcripts are degraded. In the presence of inducer, antitermination increases the synthesis of the read-through transcript. TnaC synthesis is observed in the cell-free system. However, in the presence of tryptophan, a peptidyl-tRNA also appears, TnaC-tRNA(Pro). Our findings suggest that inducer acts by preventing cleavage of TnaC peptidyl-tRNA. The ribosome associated with this newly synthesized peptidyl-tRNA presumably stalls at the tnaC stop codon, blocking Rho's access to the BoxA and rut sites, thereby preventing termination. 1-Methyltryptophan also is an effective inducer in vitro. This tryptophan analog is not incorporated into TnaC.

DNA microarray analysis of gene expression in response to physiological and genetic changes that affect tryptophan metabolism in Escherichia coli

We investigated the global changes in mRNA abundance in Escherichia coli elicited by various perturbations of tryptophan metabolism. To do so we printed DNA microarrays containing 95% of all annotated E. coli ORFs. We determined the expression profile that is predominantly dictated by the activity of the tryptophan repressor. Only three operons, trp, mtr, and aroH, exhibited appreciable expression changes consistent with this profile. The quantitative changes we observed in mRNA levels for the five genes of the trp operon were consistent within a factor of 2, with expectations based on established Trp protein levels. Several operons known to be regulated by the TyrR protein, aroF-tyrA, aroL, aroP, and aroG, were down-regulated on addition of tryptophan. TyrR can be activated by any one of the three aromatic amino acids. Only one operon, tnaAB, was significantly activated by the presence of tryptophan in the medium. We uncovered a plethora of likely indirect effects of changes in tryptophan metabolism on intracellular mRNA pools, most prominent of which was the sensitivity of arginine biosynthetic operons to tryptophan starvation.

Rho-dependent transcription termination in the tna operon of Escherichia coli: roles of the boxA sequence and the rut site

Expression of the tryptophanase (tna) operon of Escherichia coli is regulated by catabolite repression and by tryptophan-induced transcription antitermination. Tryptophan induction prevents Rho-dependent transcription termination in the leader region of the operon. Induction requires translation of a 24-residue leader peptide-coding region, tnaC, containing a single, crucial Trp codon. Studies with a lacZ reporter construct lacking the tnaC-tnaA spacer region suggest that, in the presence of excess tryptophan, the TnaC leader peptide acts in cis on the ribosome translating tnaC to inhibit its release. The stalled ribosome is thought to block Rho's access to the transcript. In this paper we examine the roles of the boxA sequence and the rut site in Rho-dependent termination. Deleting six nucleotides (CGC CCT) of boxA or introducing specific point mutations in boxA results in high-level constitutive expression. Some constitutive changes introduced in boxA do not change the TnaC peptide sequence. We confirm that deletion of the rut site results in constitutive expression. We also demonstrate that, in each constitutive construct, replacement of the tnaC start codon by a UAG stop codon reduces expression significantly, suggesting that constitutive expression requires translation of the tnaC coding sequence. Addition of bicyclomycin, an inhibitor of Rho, to these UAG constructs increases expression, demonstrating that reduced expression is due to Rho action. Combining a boxA point mutation with rut site deletion results in constitutive expression comparable to that of a maximally induced operon. These results support the hypothesis that in the presence of tryptophan the ribosome translating tnaC blocks Rho's access to the boxA and rut sites, thereby preventing transcription termination.

A Bacillus subtilis gene of previously unknown function, yhaG, is translationally regulated by tryptophan-activated TRAP and appears to be involved in tryptophan transport

Computer analysis of the Bacillus subtilis genome sequence revealed a gene with no previously attributed function, yhaG, specifying a transcript containing a presumptive binding site for the tryptophan-activated regulatory protein, TRAP. The presumptive TRAP binding site overlaps the yhaG Shine-Dalgarno sequence and translation initiation region. TRAP was shown to regulate expression of yhaG translationally. Production of the yhaG transcript in vivo was found to compete for the binding of TRAP to other known TRAP binding sites. YhaG is likely to be a transmembrane protein involved in tryptophan transport.