Recent studies examining the molecular mechanisms of isoniazid (INH) resistance in Mycobacterium tuberculosis have demonstrated that a significant percentage of drug-resistant strains are mutated in the katG gene which encodes a catalase-peroxidase, and the majority of these alterations are missense mutations which result in the substitution of a single amino acid. In previous reports, residues which may be critical for enzymatic activity and the drug-resistant phenotype have been identified by evaluating INH-resistant clinical isolates and in vitro mutants. In this study, site-directed mutagenesis techniques were utilized to alter the wild-type katG gene from M. tuberculosis at 13 of these codons. The effects of these mutations were determined using complementation assays in katG-defective, INH-resistant strains of Mycobacterium smegmatis and Mycobacterium bovis BCG. This mutational analysis revealed that point mutations in the katG gene at nine of the 13 codons can cause drug resistance, and that enzymatic activity and resistance to INH are inversely related. In addition, mutations in the mycobacterial catalase-peroxidase which reduce catalase activity also decrease peroxidase activity.

Transcription of downstream genes in the early operons of phage A requires a promoter-proximal element known as nut. This site acts in cis in the form of RNA to assemble a transcription antitermination complex which is composed of A N protein and at least four host factors. The nut-site RNA contains a small stem-loop structure called boxB. Here, we show that boxB RNA binds to N protein with high affinity and specificity. While N binding is confined to the 5' subdomain of the stem-loop, specific N recognition relies on both an intact stem-loop structure and two critical nucleotides in the pentamer loop. Substitutions of these nucleotides affect both N binding and antitermination. Remarkably, substitutions of other loop nucleotides also diminish antitermination in vivo, yet they have no detectable effect on N binding in vitro. These 3' loop mutants fail to support antitermination in a minimal system with RNA polymerase (RNAP), N, and the host factor NusA. Furthermore, the ability of NusA to stimulate the formation of the RNAP-boxB-N complex is diminished with these mutants. Hence, we suggest that boxB RNA performs two critical functions in antitermination. First, boxB binds to N and secures it near RNAP to enhance their interaction, presumably by increasing the local concentration of N. Second, boxB cooperates with NusA, most likely to bring N and RNAP in close contact and transform RNAP to the termination-resistant state.The positive control of genes that facilitate the bimodal development of A and related phages in Escherichia coli depends on two distinct operon-specific antiterminators (1). The N antiterminator activates the early operons, whereas the Q antiterminator activates the late operon. Both proteins function by a common mechanism: they capture RNA polymerase (RNAP) during early phases of transcription and mask RNAP's response to the downstream terminators (2-8). However, each antiterminator recognizes the respective genetic signal and captures RNAP by distinct mechanisms. The signals for Q action span the late promoter and the early transcribed region. Q binds to a DNA sequence within the late promoter and acts upon RNAP paused at a defined site (9). Specific nucleotides in the nontemplate strand of this region interact with RNAP not only to induce pausing but also to endow upon RNAP the conformation that is essential for engagement by Q (10). In contrast, the nut site, required for N action, functions in the form of . It can facilitate the productive interaction between N and RNAP at remote sites, suggesting that nut RNA may act similarly to DNA enhancers, binding N and delivering N to RNAP through RNA looping (11). Finally, while a single host factor (NusA) appears to be sufficient for Q activity, processive antitermination by N demands three additional factors: NusB, S10 ribosomal protein (NusE), and NusG (2, 14-16).The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §173...

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Delafloxacin demonstrates excellent antibacterial potency and exhibits a low probability for the selection of resistant mutants in MRSA. Although mutants can be selected at low frequencies in vitro from quinolone-resistant isolates, delafloxacin MICs and MPCs remain low and a fitness cost can be observed. Consequently delafloxacin warrants further investigation for the potential treatment of drug-resistant MRSA infections.

As the global threat of drug- and antibiotic-resistant bacteria continues to rise, new strategies are required to advance the drug discovery process. This work describes the construction of an array of Escherichia coli strains for use in whole-cell screens to identify new antimicrobial compounds. We used the recombination systems from bacteriophages lambda and P1 to engineer each strain in the array for low-level expression of a single, essential gene product, thus making each strain hypersusceptible to specific inhibitors of that gene target. Screening of nine strains from the array in parallel against a large chemical library permitted identification of new inhibitors of bacterial growth. As an example of the target specificity of the approach, compounds identified in the whole-cell screen for MurA inhibitors were also found to block the biochemical function of the target when tested in vitro.

During transcription of phage A early operons, the N gene product alters host RNA polymerase (RNAP) so that transcription proceeds through multiple stop signals. Here, we reproduce the essence of N activity with purified components in synthetic transcription units that contain A pL promoter and the N-recognition site, nutL, followed by a variety ofintrinsic terminators. We show that three host factors (NusA, NusE, and NusG) are essential for N to allow appreciable transcription through multiple terminators and that this persistent antitermination is stimulated by a fourth factor, NusB. Remarkably, in the absence of all four factors, N suppresses various terminators placed near the nut site. This basal antitenation activity of N is enhanced by NusA and is diminished by high salt and temperature. We postulate that N interacts with RNAP directly, inducing the terminationresistant state. While NusA facilitates this interaction, the other factors strengthen it sufficiently over time and distance so that RNAP bypasses multiple terminators. The dispensability of NusB for persistent antitermination in vitro, but not in vivo, raises the possibility that NusB performs two functions: it increases the stability of N antitermination complex and also counteracts an inhibitory factor in the cell.The RNA polymerase (RNAP) elongation complex is the target of two kinds of regulatory factors that modulate the elongation-termination decision in transcription (1-3). (4,5). Others convert RNAP to the termination-resistant form. The archetype of this latter class of antiterminators is the N gene product of phage A (6-8). The N protein is essential for transcription of the early genes of phage A, which are organized in two large operons (8). RNAP transcribing each operon encounters both factor-dependent and factor-independent terminators that are suppressed by N (9). The suppression of multiple terminators in each operon requires a specific site called nut, located between the promoter and the first terminator (10-12). The nut site acts in the form of RNA (13-15). During transcription, N is thought to bind to a small hairpin component of nut RNA, known as boxB, so that N can capture the RNAP elongation complex and transform RNAP into the termination-resistant form (7, 13-18).Antitermination by N is influenced by multiple protein factors of Escherichia coli (7,(19)(20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30). Three of these factors, NusA, NusB, and NusE (the S10 ribosomal protein), were discovered by the isolation of host mutants that are defective in N antitermination and, hence, fail to support A development (20-22). The fourth factor, NusG, emerged from the isolation ofextragenic suppressors, which permit A growth in a nusA mutant host (28). Further genetic studies indicated that these factors interact with each other, N, and RNAP (19). Indeed, N and all four host factors form a stable transcription complex in vitro (16,17,29). Although no evidence for a direct interaction between N and RNAP has been reported, N does bin...

New and improved antibiotics are urgently needed to combat the ever-increasing number of multidrugresistant bacteria. In this study, we characterized several members of a new oxazolidinone family, R-01. This antibiotic family is distinguished by having in vitro and in vivo activity against hospital-acquired, as well as community-acquired, pathogens. We compared the 50S ribosome binding affinity of this family to that of the only marketed oxazolidinone antibiotic, linezolid, using chloramphenicol and puromycin competition binding assays. The competition assays demonstrated that several members of the R-01 family displace, more effectively than linezolid, compounds known to bind to the ribosomal A site. We also monitored binding by assessing whether R-01 compounds protect U2585 (Escherichia coli numbering), a nucleotide that influences peptide bond formation and peptide release, from chemical modification by carbodiimide. The R-01 oxazolidinones were able to inhibit translation of ribosomes isolated from linezolid-resistant Staphylococcus aureus at submicromolar concentrations. This improved binding corresponds to greater antibacterial activity against linezolid-resistant enterococci. Consistent with their ribosomal A-site targeting and greater potency, the R-01 compounds promote nonsense suppression and frameshifting to a greater extent than linezolid. Importantly, the gain in potency does not impact prokaryotic specificity as, like linezolid, the members of the R-01 family show translation 50% inhibitory concentrations that are at least 100-fold higher for eukaryotic than for prokaryotic ribosomes. This new family of oxazolidinones distinguishes itself from linezolid by having greater intrinsic activity against linezolid-resistant isolates and may therefore offer clinicians an alternative to overcome linezolid resistance. A member of the R-01 family of compounds is currently undergoing clinical trials.

This work describes the novel use of tolC as a selectable/counter-selectable marker for the facile modification of DNA in Escherichia coli. Expression of TolC (an outer membrane protein) confers relative resistance to toxic small molecules, while its absence renders the cell tolerant to colicin E1. These features, coupled with the λredgam recombination system, allow for selection of tolC insertions/deletions anywhere on the E. coli chromosome or on plasmid DNA. This methodology obviates the need for minimal growth media, specialized wash protocols and the lengthy incubation times required by other published recombineering methods. As a rigorous test of the TolC selection system, six out of seven 23S rRNA genes were consecutively and seamlessly removed from the E. coli chromosome without affecting expression of neighboring genes within the complex rrn operons. The resulting plasmid-free strain retains one 23S rRNA gene (rrlC) in its natural location on the chromosome and is the first mutant of its kind. These new rRNA mutants will be useful in the study of rRNA gene regulation and ribosome function. Given its high efficiency, low background and facility in rich media, tolC selection is a broadly applicable method for the modification of DNA by recombineering.