Error-prone initiation factor 2 mutations reduce the fitness cost of antibiotic resistance

Compensatory evolution in NusG improves fitness of drug-resistant M. tuberculosis

Drug-resistant bacteria are emerging as a global threat, despite frequently being less fit than their drug-susceptible ancestors1-8. Here we sought to define the mechanisms that drive or buffer the fitness cost of rifampicin resistance (RifR) in the bacterial pathogen Mycobacterium tuberculosis (Mtb). Rifampicin inhibits RNA polymerase (RNAP) and is a cornerstone of modern short-course tuberculosis therapy9,10. However, RifR Mtb accounts for one-quarter of all deaths due to drug-resistant bacteria11,12. We took a comparative functional genomics approach to define processes that are differentially vulnerable to CRISPR interference (CRISPRi) inhibition in RifR Mtb. Among other hits, we found that the universally conserved transcription factor NusG is crucial for the fitness of RifR Mtb. In contrast to its role in Escherichia coli, Mtb NusG has an essential RNAP pro-pausing function mediated by distinct contacts with RNAP and the DNA13. We find this pro-pausing NusG-RNAP interface to be under positive selection in clinical RifR Mtb isolates. Mutations in the NusG-RNAP interface reduce pro-pausing activity and increase fitness of RifR Mtb. Collectively, these results define excessive RNAP pausing as a molecular mechanism that drives the fitness cost of RifR in Mtb, identify a new mechanism of compensation to overcome this cost, suggest rational approaches to exacerbate the fitness cost, and, more broadly, could inform new therapeutic approaches to develop drug combinations to slow the evolution of RifR in Mtb.

Genetic analysis of translation initiation in bacteria: An initiator tRNA-centric view

Translation of messenger RNA (mRNA) in bacteria occurs in the steps of initiation, elongation, termination, and ribosome recycling. The initiation step comprises multiple stages and uses a special transfer RNA (tRNA) called initiator tRNA (i-tRNA), which is first aminoacylated and then formylated using methionine and N10 -formyl-tetrahydrofolate (N10 -fTHF), respectively. Both methionine and N10 -fTHF are produced via one-carbon metabolism, linking translation initiation with active cellular metabolism. The fidelity of i-tRNA binding to the ribosomal peptidyl-site (P-site) is attributed to the structural features in its acceptor stem, and the highly conserved three consecutive G-C base pairs (3GC pairs) in the anticodon stem. The acceptor stem region is important in formylation of the amino acid attached to i-tRNA and in its initial binding to the P-site. And, the 3GC pairs are crucial in transiting the i-tRNA through various stages of initiation. We utilized the feature of 3GC pairs to investigate the nuanced layers of scrutiny that ensure fidelity of translation initiation through i-tRNA abundance and its interactions with the components of the translation apparatus. We discuss the importance of i-tRNA in the final stages of ribosome maturation, as also the roles of the Shine-Dalgarno sequence, ribosome heterogeneity, initiation factors, ribosome recycling factor, and coevolution of the translation apparatus in orchestrating a delicate balance between the fidelity of initiation and/or its leakiness to generate proteome plasticity in cells to confer growth fitness advantages in response to the dynamic nutritional states.

Suppression of Formylation Provides an Alternative Approach to Vacant Codon Creation in Bacterial In Vitro Translation

Genetic code reprogramming is a powerful approach to controlled protein modification. A remaining challenge, however, is the generation of vacant codons. We targeted the initiation machinery of E. coli, showing that restriction of the formyl donor or inhibition of the formyl transferase during in vitro translation is sufficient to prevent formylation of the acylated initiating tRNA and thereby create a vacant initiation codon that can be reprogrammed by exogenously charged tRNA. Our approach conveniently generates peptides and proteins tagged N‐terminally with non‐canonical functional groups at up to 99 % reprogramming efficiency, in combination with decoding the AUG elongation codons either with native methionine or with further reprogramming with azide‐ and alkyne‐containing cognates. We further show macrocyclization and intermolecular modifications with these click handles, thus emphasizing the applicability of our method to current challenges in peptide and protein chemistry.

Comparative proteomic analysis reveals drug resistance of Staphylococcus xylosus ATCC700404 under tylosin stress

Background

As a kind of opportunist pathogen, Staphylococcus xylosus (S. xylosus) can cause mastitis. Antibiotics are widely used for treating infected animals and tylosin is a member of such group. Thus, the continuous use of antibiotics in dairy livestock enterprise will go a long way in increasing tylosin resistance. However, the mechanism of tylosin-resistant S. xylosus is not clear. Here, isobaric tag for relative and absolute quantitation (iTRAQ)-based quantitative proteomics methods was used to find resistance-related proteins.

Results

We compared the differential expression of S. xylosus in response to tylosin stress by iTRAQ. A total of 155 proteins (59 up-regulated, 96 down-regulated) with the fold-change of >1.2 or <0.8 (p value ≤0.05) were observed between the S. xylosus treated with 1/2 MIC (0.25 μg/mL) tylosin and the untreated S. xylosus. Bioinformatic analysis revealed that these proteins play important roles in stress-response and transcription. Then, in order to verify the relationship between the above changed proteins and mechanism of tylosin-resistant S. xylosus, we induced the tylosin-resistant S. xylosus, and performed quantitative PCR analysis to verify the changes in the transcription proteins and the stress-response proteins in tylosin-resistant S. xylosus at the mRNA level. The data displayed that ribosomal protein L23 (rplw), thioredoxin(trxA) and Aldehyde dehydrogenase A(aldA-1) are up-regulated in the tylosin-resistant S. xylosus, compared with the tylosin-sensitive strains.

Conclusion

Our findings demonstrate the important of stress-response and transcription in the tylosin resistance of S. xylosus and provide an insight into the prevention of this resistance, which would aid in finding new medicines .

Electronic supplementary material

The online version of this article (10.1186/s12917-019-1959-9) contains supplementary material, which is available to authorized users.

Non-canonical Binding Site for Bacterial Initiation Factor 3 on the Large Ribosomal Subunit

Canonical translation initiation in bacteria entails the assembly of the 30S initiation complex (IC), which binds the 50S subunit to form a 70S IC. IF3, a key initiation factor, is recruited to the 30S subunit at an early stage and is displaced from its primary binding site upon subunit joining. We employed four different FRET pairs to monitor IF3 relocation after 50S joining. IF3 moves away from the 30S subunit, IF1 and IF2, but can remain bound to the mature 70S IC. The secondary binding site is located on the 50S subunit in the vicinity of ribosomal protein L33. The interaction between IF3 and the 50S subunit is largely electrostatic with very high rates of IF3 binding and dissociation. The existence of the non-canonical binding site may help explain how IF3 participates in alternative initiation modes performed directly by the 70S ribosomes, such as initiation on leaderless mRNAs or re-initiation.

A conformational switch in initiation factor 2 controls the fidelity of translation initiation in bacteria

Initiation factor (IF) 2 controls the fidelity of translation initiation by selectively increasing the rate of 50S ribosomal subunit joining to 30S initiation complexes (ICs) that carry an N-formyl-methionyl-tRNA (fMet-tRNA^fMet). Previous studies suggest that rapid 50S subunit joining involves a GTP- and fMet-tRNA^fMet-dependent "activation" of IF2, but a lack of data on the structure and conformational dynamics of 30S IC-bound IF2 has precluded a mechanistic understanding of this process. Here, using an IF2-tRNA single-molecule fluorescence resonance energy transfer signal, we directly observe the conformational switch that is associated with IF2 activation within 30S ICs that lack IF3. Based on these results, we propose a model of IF2 activation that reveals how GTP, fMet-tRNA^fMet, and specific structural elements of IF2 drive and regulate this conformational switch. Notably, we find that domain III of IF2 plays a pivotal, allosteric, role in IF2 activation, suggesting that this domain can be targeted for the development of novel antibiotics.

Lack of formylated methionyl-tRNA has pleiotropic effects on Bacillus subtilis

Bacteria initiate translation using a modified amino acid, N-formylmethionine (fMet), adapted specifically for this function. Most proteins are processed co-translationally by peptide deformylase (PDF) to remove this modification. Although PDF activity is essential in WT cells and is the target of the antibiotic actinonin, bypass mutations in the fmt gene that eliminate the formylation of Met-tRNAMet render PDF dispensable. The extent to which the emergence of fmt bypass mutations might compromise the therapeutic utility of actinonin is determined, in part, by the effects of these bypass mutations on fitness. Here, we characterize the phenotypic consequences of an fmt null mutation in the model organism Bacillus subtilis. An fmt null mutant is defective for several post-exponential phase adaptive programmes including antibiotic resistance, biofilm formation, swarming and swimming motility and sporulation. In addition, a survey of well-characterized stress responses reveals an increased sensitivity to metal ion excess and oxidative stress. These diverse phenotypes presumably reflect altered synthesis or stability of key proteins involved in these processes.

The genomic basis of adaptation to the fitness cost of rifampicin resistance in Pseudomonas aeruginosa

Antibiotic resistance carries a fitness cost that must be overcome in order for resistance to persist over the long term. Compensatory mutations that recover the functional defects associated with resistance mutations have been argued to play a key role in overcoming the cost of resistance, but compensatory mutations are expected to be rare relative to generally beneficial mutations that increase fitness, irrespective of antibiotic resistance. Given this asymmetry, population genetics theory predicts that populations should adapt by compensatory mutations when the cost of resistance is large, whereas generally beneficial mutations should drive adaptation when the cost of resistance is small. We tested this prediction by determining the genomic mechanisms underpinning adaptation to antibiotic-free conditions in populations of the pathogenic bacterium Pseudomonas aeruginosa that carry costly antibiotic resistance mutations. Whole-genome sequencing revealed that populations founded by high-cost rifampicin-resistant mutants adapted via compensatory mutations in three genes of the RNA polymerase core enzyme, whereas populations founded by low-cost mutants adapted by generally beneficial mutations, predominantly in the quorum-sensing transcriptional regulator gene lasR. Even though the importance of compensatory evolution in maintaining resistance has been widely recognized, our study shows that the roles of general adaptation in maintaining resistance should not be underestimated and highlights the need to understand how selection at other sites in the genome influences the dynamics of resistance alleles in clinical settings.

The emerging role of rectified thermal fluctuations in initiator aa-tRNA- and start codon selection during translation initiation

Decades of genetic, biochemical, biophysical, and structural studies suggest that the conformational dynamics of the translation machinery (TM), of which the ribosome is the central component, play a fundamental role in the mechanism and regulation of translation. More recently, single-molecule fluorescence resonance energy transfer (smFRET) studies have provided a unique and powerful approach for directly monitoring the real-time dynamics of the TM. Indeed, smFRET studies of the elongation stage of translation have significantly enriched our understanding of the mechanisms through which stochastic, thermally driven conformational fluctuations of the TM are exploited to drive and regulate the individual steps of translation elongation [1]. Beyond translation elongation, smFRET studies of the conformational dynamics of the initiation stage of translation offer great potential for providing mechanistic information that has thus far remained difficult or impossible to obtain using traditional methods. This is particularly true of the mechanisms through which the accuracy of initiator tRNA- and start codon selection is established during translation initiation. Given that translation initiation is a major checkpoint for regulating the translation of mRNAs, obtaining such mechanistic information holds great promise for our understanding of the translational regulation of gene expression. Here, we provide an overview of the bacterial translation initiation pathway, summarize what is known regarding the biochemical functions of the IFs, and discuss various new and exciting mechanistic insights that have emerged from several recently published smFRET studies of the mechanisms that guide initiator tRNA- and start codon selection during translation initiation. These studies provide a springboard for future investigations of the conformational dynamics of the more complex eukaryotic translation initiation pathway and mechanistic studies of the role of translational regulation of gene expression in human health and disease.

The genetic basis of the fitness costs of antimicrobial resistance: a meta-analysis approach

The evolution of antibiotic resistance carries a fitness cost, expressed in terms of reduced competitive ability in the absence of antibiotics. This cost plays a key role in the dynamics of resistance by generating selection against resistance when bacteria encounter an antibiotic-free environment. Previous work has shown that the cost of resistance is highly variable, but the underlying causes remain poorly understood. Here, we use a meta-analysis of the published resistance literature to determine how the genetic basis of resistance influences its cost. We find that on average chromosomal resistance mutations carry a larger cost than acquiring resistance via a plasmid. This may explain why resistance often evolves by plasmid acquisition. Second, we find that the cost of plasmid acquisition increases with the breadth of its resistance range. This suggests a potentially important limit on the evolution of extensive multidrug resistance via plasmids. We also find that epistasis can significantly alter the cost of mutational resistance. Overall, our study shows that the cost of antimicrobial resistance can be partially explained by its genetic basis. It also highlights both the danger associated with plasmidborne resistance and the need to understand why resistance plasmids carry a relatively low cost.

Linking system-wide impacts of RNA polymerase mutations to the fitness cost of rifampin resistance in Pseudomonas aeruginosa

Fitness costs play a key role in the evolutionary dynamics of antibiotic resistance in bacteria by generating selection against resistance in the absence of antibiotics. Although the genetic basis of antibiotic resistance is well understood, the precise molecular mechanisms linking the genetic basis of resistance to its fitness cost remain poorly characterized. Here, we examine how the system-wide impacts of mutations in the RNA polymerase (RNAP) gene rpoB shape the fitness cost of rifampin resistance in Pseudomonas aeruginosa. Rifampin resistance mutations reduce transcriptional efficiency, and this explains 76% of the variation in fitness among rpoB mutants. The pleiotropic consequence of rpoB mutations is that mutants show altered relative transcript levels of essential genes. We find no evidence that global transcriptional responses have an impact on the fitness cost of rifampin resistance as revealed by transcriptome sequencing (RNA-Seq). Global changes in the transcriptional profiles of rpoB mutants compared to the transcriptional profile of the rifampin-sensitive ancestral strain are subtle, demonstrating that the transcriptional regulatory network of P. aeruginosa is robust to the decreased transcriptional efficiency associated with rpoB mutations. On a smaller scale, we find that rifampin resistance mutations increase the expression of RNAP due to decreased termination at an attenuator upstream from rpoB, and we argue that this helps to minimize the cost of rifampin resistance by buffering against reduced RNAP activity. In summary, our study shows that it is possible to dissect the molecular mechanisms underpinning variation in the cost of rifampin resistance and highlights the importance of genome-wide buffering of relative transcript levels in providing robustness against resistance mutations.

Antibiotic resistance mutations carry fitness costs. Relative to the characteristics of their antibiotic-sensitive ancestors, resistant mutants show reduced growth rates and competitive abilities. Fitness cost plays an important role in the evolution of antibiotic resistance in the absence of antibiotics; however, the molecular mechanisms underlying these fitness costs is not well understood. We applied a systems-level approach to dissect the molecular underpinnings of the fitness costs associated with rifampin resistance in P. aeruginosa and showed that most of the variation in fitness cost can be explained by the direct effect of resistance mutations on the enzymatic activity of the mutated gene. Pleiotropic changes in transcriptional profiles are subtle at a genome-wide scale, suggesting that the gene regulatory network of P. aeruginosa is robust in the face of the direct effects of resistance mutations.

Cefotaxime resistant Escherichia coli collected from a healthy volunteer; characterisation and the effect of plasmid loss

In this study 6 CTX-M positive E. coli isolates collected during a clinical study examining the effect of antibiotic use in a human trial were analysed. The aim of the study was to analyse these isolates and assess the effect of full or partial loss of plasmid genes on bacterial fitness and pathogenicity. A DNA array was utilised to assess resistance and virulence gene carriage. Plasmids were characterised by PCR-based replicon typing and addiction system multiplex PCR. A phenotypic array and insect virulence model were utilised to assess the effect of plasmid-loss in E. coli of a large multi-resistance plasmid. All six E. coli carrying bla_CTX-M-14 were detected from a single participant and were identical by pulse field gel electrophoresis and MLST. Plasmid profiling and arrays indicated absence of a large multi-drug resistance (MDR) F-replicon plasmid carrying blaTEM, aadA4, strA, strB, dfrA17/19, sul1, and tetB from one isolate. Although this isolate partially retained the plasmid it showed altered fitness characteristics e.g. inability to respire in presence of antiseptics, similar to a plasmid-cured strain. However, unlike the plasmid-cured or plasmid harbouring strains, the survival rate for Galleria mellonella infected by the former strain was approximately 5-times lower, indicating other possible changes accompanying partial plasmid loss. In conclusion, our results demonstrated that an apparently healthy individual can harbour bla_CTX-M-14E. coli strains. In one such strain, isolated from the same individual, partial absence of a large MDR plasmid resulted in altered fitness and virulence characteristics, which may have implications in the ability of this strain to infect and any subsequent treatment.

Proteomic analysis of Streptomyces coelicolor in response to Ciprofloxacin challenge

Multi-drug tolerance is an important phenotypic property that complicates treatment of infectious diseases and reshapes drug discovery. Hence a systematic study of the origins and mechanisms of resistance shown by microorganisms is imperative. Since soil-dwelling bacteria are constantly challenged with a myriad of antibiotics, they are potential reservoirs of resistance determinants that can be mobilized into pathogens over a period of time. Elucidating the resistance mechanisms in such bacteria could help future antibiotic discoveries. This research is a preliminary study conducted to determine the effects of ciprofloxacin (CIP) on the intrinsically resistant Gram-positive soil bacterium Streptomyces coelicolor. The effect was investigated by performing 2-DE on total protein extracts of cells exposed to sub-lethal concentrations of ciprofloxacin as compared to the controls. Protein identification by MALDI-TOF/TOF revealed 24 unique differentially expressed proteins, which were statistically significant. The down-regulation of proteins involved in carbohydrate metabolism indicated a shift in the cell physiology towards a state of metabolic shutdown. Furthermore, the observed decline in protein levels involved in transcription and translation machinery, along with depletion of enzymes involved in amino acid biosynthesis and protein folding could be a cellular response to DNA damage caused by CIP, thereby minimizing the effect of defective and energetically wasteful metabolic processes. This could be crucial for the initial survival of the cells before gene level changes could come into play to ensure survival under prolonged adverse conditions. These results are a first attempt towards profiling the proteome of S. coelicolor in response to antibiotic stress. This article is part of a Special Issue entitled: Trends in Microbial Proteomics.

BIOLOGICAL SIGNIFICANCE

Soil-dwelling bacteria could serve as a reservoir of resistance determinants for clinically important bacteria. In this work, we investigated, for the first time, the differential proteomic profile of S. coelicolor cells in response to sub-inhibitory concentrations of Ciprofloxacin using 2-DE. Results indicate a shift in the cell physiology towards a state of metabolic shutdown, possibly to counter the DNA damage by ciprofloxacin. Further, up-regulation of GAPDH, RNA pol mRNA and Translation IF2 protein indicates a reprogramming of the cell for long-term survival. This study could serve as a basis for further investigations to elucidate the general mechanism by which soil bacteria exhibit resistance to fluroquinolones. This may help in developing new drug protocols and inventing novel drugs to counter resistance to this class of antibiotics in pathogenic bacteria.

Staphylococcus aureus formyl-methionyl transferase mutants demonstrate reduced virulence factor production and pathogenicity

Inhibitors of peptide deformylase (PDF) represent a new class of antibacterial agents with a novel mechanism of action. Mutations that inactivate formyl methionyl transferase (FMT), the enzyme that formylates initiator methionyl-tRNA, lead to an alternative initiation of protein synthesis that does not require deformylation and are the predominant cause of resistance to PDF inhibitors in Staphylococcus aureus. Here, we report that loss-of-function mutations in FMT impart pleiotropic effects that include a reduced growth rate, a nonhemolytic phenotype, and a drastic reduction in production of multiple extracellular proteins, including key virulence factors, such as α-hemolysin and Panton-Valentine leukocidin (PVL), that have been associated with S. aureus pathogenicity. Consequently, S. aureus FMT mutants are greatly attenuated in neutropenic and nonneutropenic murine pyelonephritis infection models and show very high survival rates compared with wild-type S. aureus. These newly discovered effects on extracellular virulence factor production demonstrate that FMT-null mutants have a more severe fitness cost than previously anticipated, leading to a substantial loss of pathogenicity and a restricted ability to produce an invasive infection.

Compensatory mutations in agrC partly restore fitness in vitro to peptide deformylase inhibitor-resistant Staphylococcus aureus

OBJECTIVES

To determine how the fitness cost of deformylase inhibitor resistance conferred by fmt mutations can be genetically compensated.

METHODS

Resistant mutants were isolated and characterized with regard to their growth rates in vitro and in neutropenic mice, MIC and DNA sequence. Faster-growing compensated mutants were isolated by serial passage in culture medium, and for a subset of the resistant and compensated mutants whole-genome sequencing was performed.

RESULTS

Staphylococcus aureus mutants resistant to the peptide deformylase inhibitor actinonin had mutations in the fmt gene that conferred high-level actinonin resistance and reduced bacterial growth rate. Compensated mutants that remained fully resistant to actinonin and showed increased growth rates appeared within 30-60 generations of growth. Whole-genome sequencing and localized DNA sequencing of mutated candidate genes showed that alterations in the gene agrC were present in the majority of compensated strains. Resistant and compensated mutants grew at similar rates as the wild-type in a mouse thigh infection model.

CONCLUSIONS

Resistance to deformylase inhibitors due to fmt mutations reduces bacterial growth rates, but these costs can be reduced by mutations in the agrC gene. Mutants defective in fmt (with or without compensatory agrC mutations) grew well in an animal model, implying that they can also cause infection in a host.

Kinetic control of translation initiation in bacteria

Translation initiation is a crucial step of protein synthesis which largely defines how the composition of the cellular transcriptome is converted to the proteome and controls the response and adaptation to environmental stimuli. The efficiency of translation of individual mRNAs, and hence the basal shape of the proteome, is defined by the structures of the mRNA translation initiation regions. Initiation efficiency can be regulated by small molecules, proteins, or antisense RNAs, underscoring its importance in translational control. Although initiation has been studied in bacteria for decades, many aspects remain poorly understood. Recent evidence has suggested an unexpected diversity of pathways by which mRNAs can be recruited to the bacterial ribosome, the importance of structural dynamics of initiation intermediates, and the complexity of checkpoints for mRNA selection. In this review, we discuss how the ribosome shapes the landscape of translation initiation by non-linear kinetic processing of the transcriptome information. We summarize the major pathways by which mRNAs enter the ribosome depending on the structure of their 5' untranslated regions, the assembly and the structure of initiation intermediates, the individual and synergistic roles of initiation factors, and the mechanisms of mRNA and initiator tRNA selection.

Activation of initiation factor 2 by ligands and mutations for rapid docking of ribosomal subunits

We previously identified mutations in the GTPase initiation factor 2 (IF2), located outside its tRNA-binding domain, compensating strongly (A-type) or weakly (B-type) for initiator tRNA formylation deficiency. We show here that rapid docking of 30S with 50S subunits in initiation of translation depends on switching 30S subunit-bound IF2 from its inactive to active form. Activation of wild-type IF2 requires GTP and formylated initiator tRNA (fMet-tRNA(i)). In contrast, extensive activation of A-type IF2 occurs with only GTP or with GDP and fMet-tRNA(i), implying a passive role for initiator tRNA as activator of IF2 in subunit docking. The theory of conditional switching of GTPases quantitatively accounts for all our experimental data. We find that GTP, GDP, fMet-tRNA(i) and A-type mutations multiplicatively increase the equilibrium ratio, K, between active and inactive forms of IF2 from a value of 4 × 10(-4) for wild-type apo-IF2 by factors of 300, 8, 80 and 20, respectively. Functional characterization of the A-type mutations provides keys to structural interpretation of conditional switching of IF2 and other multidomain GTPases.