The Staphylococcus aureus ileS gene, encoding isoleucyl-tRNA synthetase, is a member of the T-box family

New Insights into the Biological Functions of Essential TsaB/YeaZ Protein in Staphylococcus aureus

TsaB/YeaZ represents a promising target for novel antibacterial agents due to its indispensable role in bacterial survival, high conservation within bacterial species, and absence of eukaryotic homologs. Previous studies have elucidated the role of the essential staphylococcal protein, TsaB/YeaZ, in binding DNA to mediate the transcription of the ilv-leu operon, responsible for encoding key enzymes involved in the biosynthesis of branched-chain amino acids-namely isoleucine, leucine, and valine (ILV). However, the regulation of ILV biosynthesis does not account for the essentiality of TsaB/YeaZ for bacterial growth. In this study, we investigated the impact of TsaB/YeaZ depletion on bacterial morphology and gene expression profiles using electron microscopy and deep transcriptomic analysis, respectively. Our results revealed significant alterations in bacterial size and surface smoothness upon TsaB/YeaZ depletion. Furthermore, we pinpointed specific genes and enriched biological pathways significantly affected by TsaB/YeaZ during the early and middle exponential phases and early stationary phases of growth. Crucially, our research uncovered a regulatory role for TsaB/YeaZ in bacterial autolysis. These discoveries offer fresh insights into the multifaceted biological functions of TsaB/YeaZ within S. aureus.

Chronicity of high and low level mupirocin resistance in Staphylococcus aureus from 30 Indian hospitals

Mupirocin is one of the most effective topically used antibiotic for the treatment of dermatitis, nasal carriage, decolonization of methicillin susceptible Staphylococcus aureus and eradication of methicillin resistant Staphylococcus aureus. Extensive use of this antibiotic has resulted in mupirocin resistance in Staphylococcus aureus which is a matter of concern. This study was conducted to evaluate the high and low level of mupirocin resistance in Staphylococcus aureus collected from various Indian hospitals. A total of 600 samples, of which 436 were pus specimens and 164 wound site swabs were collected from 30 Indian hospitals. Disc diffusion and agar dilution methods were used to test mupirocin susceptibility in methicillin resistant Staphylococcus aureus. Out of 600 Staphylococcus aureus isolates, 176 isolates (29.33%) were found to be methicillin resistant Staphylococcus aureus (MRSA). Out of 176 non-duplicate MRSA strains, 138 isolates were found to be mupirocin sensitive, 21 isolates had high level resistance whereas 17 isolates had low level resistance to mupirocin, which contributed 78.41%, 11.93% and 9.66% respectively. Multidrug resistant susceptibility was tested for all the MRSA with Cefuroxime, Cotrimoxazole and Vancomycin antibiotics. All the high and low level resistant strain were subjected to genome screening for mupA ileS gene respectively. mupA gene was found positive in all the high level resistant strain and out of 17 low level resistant strain, 16 strain were found point mutation in V588F of ileS gene. Overall, high rate of mupirocin resistance was found in the studied samples which might be a result of indiscriminate use of mupirocin in the population of studied region. This data emphasizes the urgent need for formulation of a well-defined and regulated guidelines for mupirocin use. Moreover, continuous surveillance is needed for the use of mupirocin and routine test should be performed to detect MRSA in patients and health care personnel to prevent MRSA infections.

Exploring the possible targeting strategies of liposomes against methicillin-resistant Staphylococcus aureus (MRSA)

Multi antibiotic-resistant bacterial infections are on the rise due to the overuse of antibiotics. Methicillin-resistant Staphylococcus aureus (MRSA) is one of the pathogens listed under the category of serious threats where vancomycin remains the mainstay treatment despite the availability of various antibacterial agents. Recently, decreased susceptibility to vancomycin from clinical isolates of MRSA has been reported and has drawn worldwide attention as it is often difficult to overcome and leads to increased medical costs, mortality, and longer hospital stays. Development of antibiotic delivery systems is often necessary to improve bioavailability and biodistribution, in order to reduce antibiotic resistance and increase the lifespan of antibiotics. Liposome entrapment has been used as a method to allow higher drug dosing apart from reducing toxicity associated with drugs. The surface of the liposomes can also be designed and enhanced with drug-release properties, active targeting, and stealth effects to prevent recognition by the mononuclear phagocyte system, thus enhancing its circulation time. The present review aimed to highlight the possible targeting strategies of liposomes against MRSA bacteremia systemically while investigating the magnitude of this effect on the minimum inhibitory concentration level.

RNA Sequencing Identifies a Common Physiology in Vancomycin- and Ciprofloxacin-Tolerant Staphylococcus aureus Induced by ileS Mutations

Little is known about the mechanisms by which ileS mutations induce vancomycin tolerance in Staphylococcus aureus This study showed that transcriptome profiles were similar in vancomycin-tolerant mutants and the IleRS-inhibitor-treated parent. Notably, ileS and relA, which induce a stringent response, were upregulated. The same mechanism was responsible for cross-tolerance to vancomycin and ciprofloxacin. These findings suggest that the accumulation of uncharged isoleucyl-tRNA following ileS mutations in S. aureus was responsible for drug tolerance.

Unboxing the T-box riboswitches-A glimpse into multivalent and multimodal RNA-RNA interactions

The T-box riboswitches are widespread bacterial noncoding RNAs that directly bind specific tRNAs, sense aminoacylation on bound tRNAs, and switch conformations to control amino-acid metabolism and to maintain nutritional homeostasis. The core mechanisms of tRNA recognition, amino acid sensing, and conformational switching by the T-boxes have been recently elucidated, providing a wealth of new insights into multivalent and multimodal RNA-RNA interactions. This review dissects the structures and tRNA-recognition mechanisms by the Stem I, Stem II, and Discriminator domains, which collectively compose the T-box riboswitches. It further compares and contrasts the two classes of T-boxes that regulate transcription and translation, respectively, and integrates recent findings to derive general themes, trends, and insights into complex RNA-RNA interactions. Specifically, the T-box paradigm reveals that noncoding RNAs can interact with each other through multiple coordinated contacts, concatenation of stacked helices, and mutually induced fit. Numerous tertiary contacts, especially those emanating from strings of single-stranded purines, act in concert to reinforce long-range base-pairing and stacking interactions. These coordinated, mixed-mode contacts allow the T-box RNA to sterically sense aminoacylation on the tRNA using a bipartite steric sieve, and to couple this readout to a conformational switch mediated by tRNA-T-box stacking. Together, the insights gleaned from the T-box riboswitches inform investigations into other complex RNA structures and assemblies, development of T-box-targeted antimicrobials, and may inspire design and engineering of novel RNA sensors, regulators, and interfaces. This article is categorized under: RNA Structure and Dynamics > RNA Structure, Dynamics and Chemistry Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs Regulatory RNAs/RNAi/Riboswitches > Riboswitches.

Prevalence of Slow-Growth Vancomycin Nonsusceptibility in Methicillin-Resistant Staphylococcus aureus

We previously reported a novel phenotype of vancomycin-intermediate Staphylococcus aureus (VISA), i.e., “slow VISA,” whose colonies appear only after 72 h of incubation. Slow-VISA strains can be difficult to detect because prolonged incubation is required and the phenotype is unstable. To develop a method for detection of slow-VISA isolates, we studied 23 slow-VISA isolates derived from the heterogeneous VISA (hVISA) clinical strain Mu3. We identified single nucleotide polymorphisms (SNPs) in genes involved in various pathways which have been implicated in the stringent response, such as purine/pyrimidine synthesis, cell metabolism, and cell wall peptidoglycan synthesis. We found that mupirocin, which also induces the stringent response, caused stable expression of vancomycin resistance. On the basis of these results, we developed a method for detection of slow-VISA strains by use of 0.032 μg/ml mupirocin (Yuki Katayama, 7 March 2017, patent application PCT/JP2017/008975). Using this method, we detected 53 (15.6%) slow-VISA isolates among clinical methicillin-resistant S. aureus (MRSA) isolates. In contrast, the VISA phenotype was detected in fewer than 1% of isolates. Deep-sequencing analysis showed that slow-VISA clones are present in small numbers among hVISA isolates and proliferate in the presence of vancomycin. This slow-VISA subpopulation may account in part for the recurrence and persistence of MRSA infection.

Quality Control by Isoleucyl-tRNA Synthetase of Bacillus subtilis Is Required for Efficient Sporulation

Isoleucyl-tRNA synthetase (IleRS) is an aminoacyl-tRNA synthetase whose essential function is to aminoacylate tRNA^Ile with isoleucine. Like some other aminoacyl-tRNA synthetases, IleRS can mischarge tRNA^Ile and correct this misacylation through a separate post-transfer editing function. To explore the biological significance of this editing function, we created a ileS(T233P) mutant of Bacillus subtilis that allows tRNA^Ile mischarging while retaining wild-type Ile-tRNA^Ile synthesis activity. As seen in other species defective for aminoacylation quality control, the growth rate of the ileS(T233P) strain was not significantly different from wild-type. When the ileS(T233P) strain was assessed for its ability to promote distinct phenotypes in response to starvation, the ileS(T233P) strain was observed to exhibit a significant defect in formation of environmentally resistant spores. The sporulation defect ranged from 3-fold to 30-fold and was due to a delay in activation of early sporulation genes. The loss of aminoacylation quality control in the ileS(T233P) strain resulted in the inability to compete with a wild-type strain under selective conditions that required sporulation. These data show that the quality control function of IleRS is required in B. subtilis for efficient sporulation and suggests that editing by aminoacyl-tRNA synthetases may be important for survival under starvation/nutrient limitation conditions.

Global analysis of the impact of linezolid onto virulence factor production in S. aureus USA300

The translation inhibitor linezolid is an antibiotic of last resort against Gram-positive pathogens including methicillin resistant strains of the nosocomial pathogen Staphylococcus aureus. Linezolid is reported to inhibit production of extracellular virulence factors, but the molecular cause is unknown. To elucidate the physiological response of S. aureus to linezolid in general and the inhibition of virulence factor synthesis in particular a holistic study was performed. Linezolid was added to exponentially growing S. aureus cells and the linezolid stress response was analyzed with transcriptomics and quantitative proteomics methods. In addition, scanning and transmission electron microscopy experiments as well as fluorescence microscopy analyses of the cellular DNA and membrane were performed. As previously observed in studies on other translation inhibitors, S. aureus adapts its protein biosynthesis machinery to the reduced translation efficiency. For example the synthesis of ribosomal proteins was induced. Also unexpected results like a decline in the amount of extracellular and membrane proteins were obtained. In addition, cell shape and size changed after linezolid stress and cell division was diminished. Finally, the chromosome was condensed after linezolid stress and lost contact to the membrane. These morphological changes cannot be explained by established theories. A new hypothesis is discussed, which suggests that the reduced amount of membrane and extracellular proteins and observed defects in cell division are due to the disintegration of transertion complexes by linezolid.

Determination of essentiality and regulatory function of staphylococcal YeaZ in branched-chain amino acid biosynthesis

The staphylococcal YeaZ is highly conserved in prokaryotic cells and critical for growth of many bacterial pathogens. However, the essentiality for Staphylococcus aureus growth and the biological function of YeaZ behind its essentiality remain undefined. In this study, we created and characterized a defined Pspac-regulated yeaZ expression mutant in S. aureus and demonstrated the indispensability of YeaZ for S. aureus growth. Moreover, we conducted complementation studies, not only confirmed the requirement of YeaZ for S. aureus growth, but also revealed a similarity of essential function between staphylococcal YeaZ and its E. coli homolog. On the other hand, we explored the biological functions of YeaZ and found that YeaZ is involved in the regulation of the transcription of ilv-leu operon that encodes key enzymes responsible for the biosynthesis of the branched-chain amino acids, including isoleucine, leucine, and valine (ILV). qPCR analysis showed that the 6-fold downregulation of YeaZ dramatically elevated approximately 17- to 289-fold RNA levels of ilvD, leuA and ilvA. We further confirmed the transcriptional regulation of the ilv-leu operon by YeaZ using an ilv-promoter-lux reporter system and gel-shift assays and revealed that YeaZ is able to bind the promoter region of ilv. Furthermore, we established that the regulation of ILV biosynthesis isn't associated with YeaZ's essentiality, as the deletion of the ilv-leu operon did not affect the requirement of YeaZ for growth in culture. Our results demonstrate the essentiality of YeaZ for S. aureus growth and suggest that the staphylococcal YeaZ possesses regulatory function.

The T box riboswitch: A novel regulatory RNA that utilizes tRNA as its ligand

The T box riboswitch is a cis-acting regulatory RNA that controls expression of amino acid-related genes in response to the aminoacylation state of a specific tRNA. Multiple genes in the same organism can utilize this mechanism, with each gene responding independently to its cognate tRNA. The uncharged tRNA interacts directly with the regulatory RNA element, and this interaction promotes readthrough of an intrinsic transcriptional termination site upstream of the regulated coding sequence. A second class of T box elements uses a similar tRNA-dependent response to regulate translation initiation. This review will describe the current state of our knowledge about this regulatory system. This article is part of a Special Issue entitled: Riboswitches.

Global analysis of the Staphylococcus aureus response to mupirocin

In the present study, we analyzed the response of S. aureus to mupirocin, the drug of choice for nasal decolonization. Mupirocin selectively inhibits the bacterial isoleucyl-tRNA synthetase (IleRS), leading to the accumulation of uncharged isoleucyl-tRNA and eventually the synthesis of (p)ppGpp. The alarmone (p)ppGpp induces the stringent response, an important global transcriptional and translational control mechanism that allows bacteria to adapt to nutritional deprivation. To identify proteins with an altered synthesis pattern in response to mupirocin treatment, we used the highly sensitive 2-dimensional gel electrophoresis technique in combination with mass spectrometry. The results were complemented by DNA microarray, Northern blot, and metabolome analyses. Whereas expression of genes involved in nucleotide biosynthesis, DNA metabolism, energy metabolism, and translation was significantly downregulated, expression of isoleucyl-tRNA synthetase, the branched-chain amino acid pathway, and genes with functions in oxidative-stress resistance (ahpC and katA) and putative roles in stress protection (the yvyD homologue SACOL0815 and SACOL1759 and SACOL2131) and transport processes was increased. A comparison of the regulated genes to known regulons suggests the involvement of the global regulators CodY and SigB in shaping the response of S. aureus to mupirocin. Of particular interest was the induced transcription of genes encoding virulence-associated regulators (i.e., arlRS, saeRS, sarA, sarR, sarS, and sigB), as well as genes directly involved in the virulence of S. aureus (i.e., fnbA, epiE, epiG, and seb).

Global transcriptional responses of Bacillus subtilis to xenocoumacin 1

AIMS

To determine the global transcriptional response of Bacillus subtilis to an antimicrobial agent, xenocoumacin 1 (Xcn1).

METHODS AND RESULTS

Subinhibitory concentration of Xcn1 applied to B. subtilis was measured according to Hutter's method for determining optimal concentrations. cDNA microarray technology was used to study the global transcriptional response of B. subtilis to Xcn1. Real-time RT-PCR was employed to verify alterations in the transcript levels of six genes. The subinhibitory concentration was determined to be 1 μg ml(-1). The microarray data demonstrated that Xcn1 treatment of B. subtilis led to more than a 2.0-fold up-regulation of 480 genes and more than a 2.0-fold down-regulation of 479 genes (q ≤ 0.05).

CONCLUSIONS

The transcriptional responses of B. subtilis to Xcn1 were determined, and several processes were affected by Xcn1. Additionally, cluster analysis of gene expression profiles after treatment with Xcn1 or 37 previously studied antibiotics indicated that Xcn1 has similar mechanisms of action to protein synthesis inhibitors.

SIGNIFICANCE AND IMPACT OF THE STUDY

These microarray data showed alterations of gene expression in B. subtilis after exposure to Xcn1. From the results, we identified various processes affected by Xcn1. This study provides a whole-genome perspective to elucidate the action of Xcn1 as a potential antimicrobial agent.

Transcriptional regulation of histidine biosynthesis genes in Corynebacterium glutamicum

Corynebacterium glutamicum , a gram-positive bacterium, has been widely used for industrial amino acid production. Corynebacterium glutamicum his genes are located and transcribed in two unlinked loci, hisEG and hisDCB–orf1–orf2–hisHA–impA–hisFI. The latter his operon starts the transcription at the C residue localized 196 bp upstream of the hisD ATG start codon. Our computer-based sequence analysis showed that the region corresponding to the untranslated 5′ end of the transcript, named the hisD leader region, displays the typical features of the T-box transcriptional attenuation mechanism. Therefore, expression of the cat reporter gene under the control of the wild-type or mutated hisD leader regions was tested in multi-copy (pProm and pTer series) and in single-copy (pInt series) systems under conditions of sufficient or limited histidine. Our mutational studies led to the conclusion that the CAU histidine specifier and 5′-UGGA-3′ sequence in the hisD leader region are required for the hisDCB–orf1–orf2–hisHA–impA–hisFI gene regulation. The cat gene expression from the wild-type leader region was negatively regulated by histidine. However, the cat gene expression from mutated leader regions was irresponsive to the level of histidine in the growth medium. Taken together, we propose that a T-box mediated attenuation mechanism is responsible for the gene expression of the hisDCB–orf1–orf2–hisHA–impA–hisFI operon in C. glutamicum.

An in silico analysis of T-box regulated genes and T-box evolution in prokaryotes, with emphasis on prediction of substrate specificity of transporters

BACKGROUND

T-box anti-termination is an elegant and sensitive mechanism by which many bacteria maintain constant levels of amino acid-charged tRNAs. The amino acid specificity of the regulatory element is related to a so-called specifier codon and can in principle be used to guide the functional annotation of the genes controlled via the T-box anti-termination mechanism.

RESULTS

Hidden Markov Models were defined to search the T-box regulatory element and were applied to all completed prokaryotic genomes. The vast majority of the genes found downstream of the retrieved elements encoded functionalities related to transport and synthesis of amino acids and the charging of tRNA. This is completely in line with findings reported in literature and with the proposed biological role of the regulatory element. For several species, the functional annotation of a large number of genes encoding proteins involved in amino acid transport could be improved significantly on basis of the amino acid specificity of the identified T-boxes. In addition, these annotations could be extrapolated to a larger number of orthologous systems in other species. Analysis of T-box distribution confirmed that the element is restricted predominantly to species of the phylum Firmicutes. Furthermore, it appeared that the distribution was highly species specific and that in the case of amino acid transport some boxes seemed to "pop-up" only recently.

CONCLUSION

We have demonstrated that the identification of the molecular specificity of a regulatory element can be of great help in solving notoriously difficult annotation issues, e.g. by defining the substrate specificity of genes encoding amino acid transporters on basis of the amino acid specificity of the regulatory T-box. Furthermore, our analysis of the species-dependency of the occurrence of specific T-boxes indicated that these regulatory elements propagate in a semi-independent way from the genes that they control.

Comparative genomic analysis of T-box regulatory systems in bacteria

T-box antitermination is one of the main mechanisms of regulation of genes involved in amino acid metabolism in Gram-positive bacteria. T-box regulatory sites consist of conserved sequence and RNA secondary structure elements. Using a set of known T-box sites, we constructed the common pattern and used it to scan available bacterial genomes. New T-boxes were found in various Gram-positive bacteria, some Gram-negative bacteria (delta-proteobacteria), and some other bacterial groups (Deinococcales/Thermales, Chloroflexi, Dictyoglomi). The majority of T-box-regulated genes encode aminoacyl-tRNA synthetases. Two other groups of T-box-regulated genes are amino acid biosynthetic genes and transporters, as well as genes with unknown function. Analysis of candidate T-box sites resulted in new functional annotations. We assigned the amino acid specificity to a large number of candidate amino acid transporters and a possible function to amino acid biosynthesis genes. We then studied the evolution of the T-boxes. Analysis of the constructed phylogenetic trees demonstrated that in addition to the normal evolution consistent with the evolution of regulated genes, T-boxes may be duplicated, transferred to other genes, and change specificity. We observed several cases of recent T-box regulon expansion following the loss of a previously existing regulatory system, in particular, arginine regulon in Clostridium difficile and methionine regulon in Lactobacillaceae. Finally, we described a new structural class of T-boxes containing duplicated terminator-antiterminator elements and unusual reduced T-boxes regulating initiation of translation in the Actinobacteria.

Regulog analysis: detection of conserved regulatory networks across bacteria: application to Staphylococcus aureus

A transcriptional regulatory network encompasses sets of genes (regulons) whose expression states are directly altered in response to an activating signal, mediated by trans-acting regulatory proteins and cis-acting regulatory sequences. Enumeration of these network components is an essential step toward the creation of a framework for systems-based analysis of biological processes. Profile-based methods for the detection of cis-regulatory elements are often applied to predict regulon members, but they suffer from poor specificity. In this report we describe Regulogger, a novel computational method that uses comparative genomics to eliminate spurious members of predicted gene regulons. Regulogger produces regulogs, sets of coregulated genes for which the regulatory sequence has been conserved across multiple organisms. The quantitative method assigns a confidence score to each predicted regulog member on the basis of the degree of conservation of protein sequence and regulatory mechanisms. When applied to a reference collection of regulons from Escherichia coli, Regulogger increased the specificity of predictions up to 25-fold over methods that use cis-element detection in isolation. The enhanced specificity was observed across a wide range of biologically meaningful parameter combinations, indicating a robust and broad utility for the method. The power of computational pattern discovery methods coupled with Regulogger to unravel transcriptional networks was demonstrated in an analysis of the genome of Staphylococcus aureus. A total of 125 regulogs were found in this organism, including both well-defined functional groups and a subset with unknown functions.

Regulation of biosynthesis and transport of aromatic amino acids in low-GC Gram-positive bacteria

Computational comparative techniques were applied to analysis of the aromatic amino acid regulon in Gram-positive bacteria. A new candidate transcription regulation signal of 3-deoxy-D-arabino-heptulosonate-7-phosphate synthase and shikimate kinase genes was identified in Streptococcus and Lactococcus species. New T-boxes were found upstream of aromatic amino acid biosynthesis and transport genes in the Bacillus/Clostridium group. The substrate specificity of proteins from the PabA/TrpG family was assigned based on metabolic reconstruction and analysis of regulatory signals and phylogenetic patterns. New candidate tryptophan transporters were identified; their specificity was predicted by analysis of T-box regulatory sites. Comparison of all available genomes shows that regulation of genes of the aromatic amino acid biosynthesis pathway is quite labile and involves at least four regulatory systems, two at the DNA level and two more involving competition of alternative RNA secondary structures for transcription and/or translation regulation at the RNA level.

Transcriptional regulation and signature patterns revealed by microarray analyses of Streptococcus pneumoniae R6 challenged with sublethal concentrations of translation inhibitors

The effects of sublethal concentrations of four different classes of translation inhibitors (puromycin, tetracycline, chloramphenicol, and erythromycin) on global transcription patterns of Streptococcus pneumoniae R6 were determined by microarray analyses. Consistent with the general mode of action of these inhibitors, relative transcript levels of genes that encode ribosomal proteins and translation factors or that mediate tRNA charging and amino acid biosynthesis increased or decreased, respectively. Transcription of the heat shock regulon was induced only by puromycin or streptomycin treatment, which lead to truncation or mistranslation, respectively, but not by other antibiotics that block translation, transcription, or amino acid charging of tRNA. In contrast, relative transcript amounts of certain genes involved in transport, cellular processes, energy metabolism, and purine nucleotide (pur) biosynthesis were changed by different translation inhibitors. In particular, transcript amounts from a pur gene cluster and from purine uptake and salvage genes were significantly elevated by several translation inhibitors, but not by antibiotics that target other cellular processes. Northern blotting confirmed increased transcript amounts from part of the pur gene cluster in cells challenged by translation inhibitors and revealed the presence of a 10-kb transcript. Purine metabolism genes were negatively regulated by a homologue of the PurR regulatory protein, and full derepression in a DeltapurR mutant depended on optimal translation. Unexpectedly, hierarchical clustering of the microarray data distinguished among the global transcription patterns caused by antibiotics that inhibit different steps in the translation cycle. Together, these results show that there is extensive control of transcript amounts by translation in S. pneumoniae, especially for de novo purine nucleotide biosynthesis. In addition, these global transcription patterns form a signature that can be used to classify the mode of action and potential mechanism of new translation inhibitors.

Sensing small molecules by nascent RNA: a mechanism to control transcription in bacteria

Thiamin and riboflavin are precursors of essential coenzymes-thiamin pyrophosphate (TPP) and flavin mononucleotide (FMN)/flavin adenine dinucleotide (FAD), respectively. In Bacillus spp, genes responsible for thiamin and riboflavin biosynthesis are organized in tightly controllable operons. Here, we demonstrate that the feedback regulation of riboflavin and thiamin genes relies on a novel transcription attenuation mechanism. A unique feature of this mechanism is the formation of specific complexes between a conserved leader region of the cognate RNA and FMN or TPP. In each case, the complex allows the termination hairpin to form and interrupt transcription prematurely. Thus, sensing small molecules by nascent RNA controls transcription elongation of riboflavin and thiamin operons and possibly other bacterial operons as well.

relA-Independent amino acid starvation response network of Streptococcus pyogenes

Streptococcus pyogenes (group A streptococcus [GAS]), a multiple-amino-acid-auxotrophic human pathogen, may face starvation for essential amino acids during various stages of the infection process. Since the response of GAS to such conditions is likely to influence pathogenetic processes, we set out to identify by transcriptional analyses genes and operons that are responsive to amino acid starvation and examined whether functionally meaningful response patterns can be ascertained. We discovered that GAS are capable of mounting a relA-independent amino acid starvation response that involves transcriptional modulation of a wide array of housekeeping genes as well as accessory and dedicated virulence genes. Housekeeping genes that were upregulated during starvation of both wild-type and relA mutant strains included the newly identified T-box members of the aminoacyl-tRNA synthetase genes, the genes for components of the tmRNA-mediated peptide tagging and proteolysis system for abnormal proteins (ssrA, smpB, clpP, and clpC), and the operons for the dnaK and groE groups of molecular chaperones. In addition to upregulation of the genes for oligopeptide permease (opp), intracellular peptidase (pepB), and the two-component regulator covRS reported previously (K. Steiner and H. Malke, Mol. Microbiol. 38:1004-1016, 2000), amino acid starvation stimulated the transcription of the growth phase-associated, virulence-regulatory fas operon, the streptolysin S operon (sag), and the gene for autoinducer-2 production protein (luxS). A prominent feature of operons exhibiting internal transcriptional termination (opp, fas, and sag) was starvation-promoted full-length transcription, a mechanism that improves the efficacy of these systems by increasing the level of coordinate transcription of functionally related genes. Based on these results, a regulatory network with feedback mechanisms is proposed that counteracts the stringent response, links the levels of key rate-limiting enzymes to virulence gene expression, and enables the organism in a dynamic way to take advantage of protein-rich environments provided by its human host. As several of the affected target genes are controlled by more than one regulator, fine modulation may result in accordance with the demands imposed by ecologically different colonization sites upon the adaptive capacity of the pathogen.

Transcription termination control in bacteria

Transcription termination is a dynamic process and is subject to control at a number of levels. New information about the molecular mechanisms of transcription elongation and termination, as well as new insights into protein-RNA interactions, are providing a framework for increased understanding of the molecular details of transcription termination control.

Regulation of expression of the Lactococcus lactis histidine operon

In Lactococcus lactis, the his operon contains all the genes necessary for histidine biosynthesis. It is transcribed from a unique promoter, localized 300 bp upstream of the first gene. The region corresponding to the untranslated 5' end of the transcript, named the his leader region, displays the typical features of the T box transcriptional attenuation mechanism which is involved in the regulation of many amino acid biosynthetic operons and tRNA synthetase genes in gram-positive bacteria. Here we describe the regulation of transcription of the his operon by the level of histidine in the growth medium. In the absence of histidine, two transcripts are present. One covers the entire operon, while the other stops at a terminator situated about 250 bp downstream of the transcription start point. DNA sequences implicated in regulation of the his operon were identified by transcriptional fusion with luciferase genes and site-directed mutagenesis. In addition to the previously defined sequences necessary for effective T-box-mediated regulation, new essential regions were identified. Eighteen percent of the positions of the his leader region were found to differ in seven distantly related strains of L. lactis. Analysis of the variable positions supports the folding model of the central part of the his leader region. Lastly, in addition to the T-box-mediated regulation, the operon is regulated at the level of initiation of transcription, which is repressed in the presence of histidine. An operator site, necessary for full repression, overlaps the terminator involved in the T box attenuation mechanism. The functionality of the operator is altered on plasmids with low and high copy numbers, suggesting that supercoiling may play a role in the expression of the his operon. The extents of regulation at the levels of initiation and attenuation of transcription are 6- to 8-fold and 14-fold, respectively. Together, the two levels of control allow a 120-fold range of regulation of the L. lactis operon by histidine.

In vitro and in vivo secondary structure probing of the thrS leader in Bacillus subtilis

The Bacillus subtilis thrS gene is a member of the T-box gene family in Gram-positive organisms whose expression is regulated by a tRNA-mediated transcriptional antitermination mechanism involving a direct tRNA:mRNA interaction. The complex leader sequences of these genes share only short stretches of primary sequence homology, but a common secondary structure has been proposed by comparing the leaders of many genes of this family. The proposed mechanism forthe tRNA:mRNA interaction depends heavily on the secondary structure model, but is so far only supported by genetic evidence. We have studied the structure of the B.subtilis thrS leader in solution, in protection experiments using both chemical and enzymatic probes. The thrS leader structure was also probed in vivo using dimethylsulphate and the in vitro and in vivo data are in good accordance. We have organized the thrS leader into three major domains comprising six separate stem-loops. All but one of the short sequences conserved in this gene family are present in loop structures. The ACC specifier codon proposed to interact with the tRNAThrGGUisoacceptor is present in a bulge and probably exists in a stacking conformation. The proposed antiterminator structure is not visible in transcripts containing the terminator, but was probed using a transcript with the 3'-half of the terminator deleted and its folding appears consistent with the regulatory model. The leader sequences, and in particular the specifier domains, of the other genes of this family can be folded similarly to the experimentally solved thrS structure.

Unconventional organization of the division and cell wall gene cluster of Streptococcus pneumoniae

The genes responsible for cell wall biosynthesis and cell division (dcw genes) were identified and sequenced in Streptococcus pneumoniae. The genetic organization of the dcw cluster in Streptococcus pneumoniae differed significantly from the clusters of other bacteria reported to date. In particular, the genes corresponding to the 2 min region of the Escherichia coli chromosome were found distributed in three genetically separate regions of the Streptococcus pneumoniae chromosome. The first region contained the expected ftsA and ftsZ cell division genes at one end and pbp2b, ddl and murF at the other end. The murD, murG and divIB genes, always found located upstream of ftsA, were found in a second region separated from the first. A third region contained the yllC, yllD, pbp2x and mraY genes. The chromosomal region downstream of ftsZ was also sequenced and characterized. In Streptococcus pneumoniae this region contains four ORFs, all of unknown function, and an ORF encoding the Bacillus subtilis DivIVA homologue. The gene order and the organization of this region was found to be conserved in Staphylococcus aureus, Streptococcus pyogenes and Bacillus subtilis, raising the possibility that previously unidentified loci may also be involved in division.

Analysis and overexpression in Escherichia coli of a staphylococcal gene encoding seryl-tRNA synthetase

We have sequenced and expressed in Escherichia coli the gene encoding the seryl-tRNA synthetase from the pathogenic bacterium Staphylococcus aureus. The overexpressed and purified recombinant enzyme was able to aminoacylate unfractionated tRNA from E. coli. Its activity was not affected by antibodies raised against and inhibiting the E. coli seryl-tRNA synthetase.