Molecular Mechanism of Drug-Dependent Ribosome Stalling

Integron-Derived Aminoglycoside-Sensing Riboswitches Control Aminoglycoside Acetyltransferase Resistance Gene Expression

Class 1 integrons accumulate antibiotic resistance genes by site-specific recombination at aatI-1 sites. Captured genes are transcribed from a promoter located within the integron; for class 1 integrons, the first gene to be transcribed and translated normally encodes an aminoglycoside antibiotic resistance protein (either an acetyltransferase [AAC] or adenyltransferase [AAD]). The leader RNA from the Pseudomonas fluorescens class 1 integron contains an aminoglycoside-sensing riboswitch RNA that controls the expression of the downstream aminoglycoside resistance gene. Here, we explore the relationship between integron-dependent DNA recombination and potential aminoglycoside-sensing riboswitch products of recombination derived from a series of aminoglycoside-resistant clinical strains. Sequence analysis of the clinical strains identified a series of sequence variants that were associated with class I integron-derived aminoglycoside-resistant (both aac and aad) recombinants. For the aac recombinants, representative sequences showed up to 6-fold aminoglycoside-dependent regulation of reporter gene expression. Microscale thermophoresis (MST) confirmed RNA binding. Covariance analysis generated a secondary-structure model for the RNA that is an independent verification of previous models that were derived from mutagenesis and chemical probing data and that was similar to that of the P. fluorescens riboswitch RNA. The aminoglycosides were among the first antibiotics to be used clinically, and the data suggest that in an aminoglycoside-rich environment, functional riboswitch recombinants were selected during integron-mediated recombination to regulate aminoglycoside resistance. The incorporation of a functional aminoglycoside-sensing riboswitch by integron recombination confers a selective advantage for the expression of resistance genes of diverse origins.

Using the Bacterial Ribosome as a Discovery Platform for Peptide-Based Antibiotics

The threat of bacteria resistant to multiple antibiotics poses a major public health problem requiring immediate and coordinated action worldwide. While infectious pathogens have become increasingly resistant to commercially available drugs, antibiotic discovery programs in major pharmaceutical companies have produced no new antibiotic scaffolds in 40 years. As a result, new strategies must be sought to obtain a steady supply of novel scaffolds capable of countering the spread of resistance. The bacterial ribosome is a major target for antimicrobials and is inhibited by more than half of the antibiotics used today. Recent studies showing that the ribosome is a target for several classes of ribosomally synthesized antimicrobial peptides point to ribosome-targeting peptides as a promising source of antibiotic scaffolds. In this Perspective, we revisit the current paradigm of antibiotic discovery by proposing that the bacterial ribosome can be used both as a target and as a tool for the production and selection of peptide-based antimicrobials. Turning the ribosome into a high-throughput platform for the directed evolution of peptide-based antibiotics could be achieved in different ways. One possibility would be to use a combination of state-of-the-art microfluidics and genetic reprogramming techniques, which we will review briefly. If it is successful, this strategy has the potential to produce new classes of antibiotics for treating multi-drug-resistant pathogens.

Odilorhabdins, Antibacterial Agents that Cause Miscoding by Binding at a New Ribosomal Site

Growing resistance of pathogenic bacteria and shortage of antibiotic discovery platforms challenge the use of antibiotics in the clinic. This threat calls for exploration of unconventional sources of antibiotics and identification of inhibitors able to eradicate resistant bacteria. Here we describe a different class of antibiotics, odilorhabdins (ODLs), produced by the enzymes of the non-ribosomal peptide synthetase gene cluster of the nematode-symbiotic bacterium Xenorhabdus nematophila. ODLs show activity against Gram-positive and Gram-negative pathogens, including carbapenem-resistant Enterobacteriaceae, and can eradicate infections in animal models. We demonstrate that the bactericidal ODLs interfere with protein synthesis. Genetic and structural analyses reveal that ODLs bind to the small ribosomal subunit at a site not exploited by current antibiotics. ODLs induce miscoding and promote hungry codon readthrough, amino acid misincorporation, and premature stop codon bypass. We propose that ODLs' miscoding activity reflects their ability to increase the affinity of non-cognate aminoacyl-tRNAs to the ribosome.

Unveiling the structural features that determine the dual methyltransferase activities of Streptococcus pneumoniae RlmCD

Methyltransferase RlmCD was previously shown to be responsible for the introduction of C5 methylation at both U747 and U1939 of the 23S ribosomal RNA in Streptococcus pneumoniae. Intriguingly, its structural homologue, RumA, can only catalyze the methylation of U1939, while RlmC is the dedicated enzyme for m5U747 in Escherichia coli. In this study, we describe the structure of RlmCD in complex with its cofactor and the RNA substrate containing U747 at 2.00 Å or U1939 at 3.10 Å. We demonstrate that multiple structural features collaborate to establish the dual enzymatic activities of RlmCD. Of them, the side-chain rearrangement of F145 was observed to be an unusual mechanism through which RlmCD can discriminate between U747- and U1939-containing RNA substrate by switching the intermolecular aromatic stacking between protein and RNA on/off. An in-vitro methyltransferase assay and electrophoretic mobility shift assay were performed to validate these findings. Overall, our complex structures allow for a better understanding of the dual-functional mechanism of RlmCD, suggesting useful implications for the evolution of the RumA-type enzyme and the potential development of antibiotic drugs against S. pneumoniae.

Exploring allosteric communication in multiple states of the bacterial ribosome using residue network analysis

Antibiotic resistance is one of the most important problems of our era and hence the discovery of new effective therapeutics is urgent. At this point, studying the allosteric communication pathways in the bacterial ribosome and revealing allosteric sites/residues is critical for designing new inhibitors or repurposing readily approved drugs for this enormous machine. To shed light onto molecular details of the allosteric mechanisms, here we construct residue networks of the bacterial ribosomal complex at four different states of translation by using an effective description of the intermolecular interactions. Centrality analysis of these networks highlights the functional roles of structural components and critical residues on the ribosomal complex. High betweenness scores reveal pathways of residues connecting numerous sites on the structure. Interestingly, these pathways assemble highly conserved residues, drug binding sites, and known allosterically linked regions on the same structure. This study proposes a new residue-level model to test how distant sites on the molecular machine may be linked through hub residues that are critically located on the contact topology to inherently form communication pathways. Findings also indicate intersubunit bridges B1b, B3, B5, B7, and B8 as critical targets to design novel antibiotics.

Ribosomal stalling landscapes revealed by high-throughput inverse toeprinting of mRNA libraries

High-throughput inverse toeprinting identifies peptide-encoding transcripts that induce ribosome stalling and allows the systematic analysis of sequence-dependent translational events.

Although it is known that the amino acid sequence of a nascent polypeptide can impact its rate of translation, dedicated tools to systematically investigate this process are lacking. Here, we present high-throughput inverse toeprinting, a method to identify peptide-encoding transcripts that induce ribosomal stalling in vitro. Unlike ribosome profiling, inverse toeprinting protects the entire coding region upstream of a stalled ribosome, making it possible to work with random or focused transcript libraries that efficiently sample the sequence space. We used inverse toeprinting to characterize the stalling landscapes of free and drug-bound Escherichia coli ribosomes, obtaining a comprehensive list of arrest motifs that were validated in vivo, along with a quantitative measure of their pause strength. Thanks to the modest sequencing depth and small amounts of material required, inverse toeprinting provides a highly scalable and versatile tool to study sequence-dependent translational processes.

Context-Specific Action of Ribosomal Antibiotics

The ribosome is a major antibiotic target. Many types of inhibitors can stop cells from growing by binding at functional centers of the ribosome and interfering with its ability to synthesize proteins. These antibiotics were usually viewed as general protein synthesis inhibitors, which indiscriminately stop translation at every codon of every mRNA, preventing the ribosome from making any protein. However, at each step of the translation cycle, the ribosome interacts with multiple ligands (mRNAs, tRNA substrates, translation factors, etc.), and as a result, the properties of the translation complex vary from codon to codon and from gene to gene. Therefore, rather than being indiscriminate inhibitors, many ribosomal antibiotics impact protein synthesis in a context-specific manner. This review presents a snapshot of the growing body of evidence that some, and possibly most, ribosome-targeting antibiotics manifest site specificity of action, which is modulated by the nature of the nascent protein, the mRNA, or the tRNAs.

How Macrolide Antibiotics Work

Macrolide antibiotics inhibit protein synthesis by targeting the bacterial ribosome. They bind at the nascent peptide exit tunnel and partially occlude it. Thus, macrolides have been viewed as 'tunnel plugs' that stop the synthesis of every protein. More recent evidence, however, demonstrates that macrolides selectively inhibit the translation of a subset of cellular proteins, and that their action crucially depends on the nascent protein sequence and on the antibiotic structure. Therefore, macrolides emerge as modulators of translation rather than as global inhibitors of protein synthesis. The context-specific action of macrolides is the basis for regulating the expression of resistance genes. Understanding the details of the mechanism of macrolide action may inform rational design of new drugs and unveil important principles of translation regulation.

How Messenger RNA and Nascent Chain Sequences Regulate Translation Elongation

Translation elongation is a highly coordinated, multistep, multifactor process that ensures accurate and efficient addition of amino acids to a growing nascent-peptide chain encoded in the sequence of translated messenger RNA (mRNA). Although translation elongation is heavily regulated by external factors, there is clear evidence that mRNA and nascent-peptide sequences control elongation dynamics, determining both the sequence and structure of synthesized proteins. Advances in methods have driven experiments that revealed the basic mechanisms of elongation as well as the mechanisms of regulation by mRNA and nascent-peptide sequences. In this review, we highlight how mRNA and nascent-peptide elements manipulate the translation machinery to alter the dynamics and pathway of elongation.

Ribosome protection by antibiotic resistance ATP-binding cassette protein

ARE ABC-F genes have been found in numerous pathogen genomes and multi-drug resistance conferring plasmids. Further transmission will challenge the clinical use of many antibiotics. The development of improved ribosome-targeting therapeutics relies on the elucidation of the resistance mechanisms. Characterization of MsrE protein bound to the bacterial ribosome is first of its kind for ARE ABC-F members. Together with biochemical data, it sheds light on the ribosome protection mechanism by domain linker-mediated conformational change and displacement leading to drug release, suggesting a mechanism shared by other ARE ABC-F proteins. These proteins present an intriguing example of structure-function relationship and a medically relevant target of study as they collectively mediate resistance to the majority of antibiotic classes targeting the peptidyl-transferase center region.

The ribosome is one of the richest targets for antibiotics. Unfortunately, antibiotic resistance is an urgent issue in clinical practice. Several ATP-binding cassette family proteins confer resistance to ribosome-targeting antibiotics through a yet unknown mechanism. Among them, MsrE has been implicated in macrolide resistance. Here, we report the cryo-EM structure of ATP form MsrE bound to the ribosome. Unlike previously characterized ribosomal protection proteins, MsrE is shown to bind to ribosomal exit site. Our structure reveals that the domain linker forms a unique needle-like arrangement with two crossed helices connected by an extended loop projecting into the peptidyl-transferase center and the nascent peptide exit tunnel, where numerous antibiotics bind. In combination with biochemical assays, our structure provides insight into how MsrE binding leads to conformational changes, which results in the release of the drug. This mechanism appears to be universal for the ABC-F type ribosome protection proteins.

Erythromycin leads to differential protein expression through differences in electrostatic and dispersion interactions with nascent proteins

The antibiotic activity of erythromycin, which reversibly binds to a site within the bacterial ribosome exit tunnel, against many gram positive microorganisms indicates that it effectively inhibits the production of proteins. Similar to other macrolides, the activity of erythromycin is far from universal, as some peptides can bypass the macrolide-obstructed exit tunnel and become partially or fully synthesized. It is unclear why, at the molecular level, some proteins can be synthesized while others cannot. Here, we use steered molecular dynamics simulations to examine how erythromycin inhibits synthesis of the peptide ErmCL but not the peptide H-NS. By pulling these peptides through the exit tunnel of the E.coli ribosome with and without erythromycin present, we find that erythromycin directly interacts with both nascent peptides, but the force required for ErmCL to bypass erythromycin is greater than that of H-NS. The largest forces arise three to six residues from their N-terminus as they start to bypass Erythromycin. Decomposing the interaction energies between erythromycin and the peptides at this point, we find that there are stronger electrostatic and dispersion interactions with the more C-terminal residues of ErmCL than with H-NS. These results suggest that erythromycin slows or stalls synthesis of ErmCL compared to H-NS due to stronger interactions with particular residue positions along the nascent protein.

Methylated mRNA Nucleotides as Regulators for Ribosomal Translation

Methylated RNA nucleotides were recently discovered to be highly abundant in RNAs. The effects of these methylations were mainly attributed to altered mRNA stabilities, protein-binding affinities, or RNA structures. The direct impact of RNA modifications on the performance of the ribosome has not been investigated so far. In this chapter, we describe an approach that allows introducing RNA modifications site-specifically into coding sequences of mRNAs and determining their effect on the translation machinery in a well-defined bacterial in vitro system.

Binding and Action of Amino Acid Analogs of Chloramphenicol upon the Bacterial Ribosome

Antibiotic chloramphenicol (CHL) binds with a moderate affinity at the peptidyl transferase center of the bacterial ribosome and inhibits peptide bond formation. As an approach for modifying and potentially improving properties of this inhibitor, we explored ribosome binding and inhibitory activity of a number of amino acid analogs of CHL. The L-histidyl analog binds to the ribosome with the affinity exceeding that of CHL by 10 fold. Several of the newly synthesized analogs were able to inhibit protein synthesis and exhibited the mode of action that was distinct from the action of CHL. However, the inhibitory properties of the semi-synthetic CHL analogs did not correlate with their affinity and in general, the amino acid analogs of CHL were less active inhibitors of translation in comparison with the original antibiotic. The X-ray crystal structures of the Thermus thermophilus 70S ribosome in complex with three semi-synthetic analogs showed that CHL derivatives bind at the peptidyl transferase center, where the aminoacyl moiety of the tested compounds established idiosyncratic interactions with rRNA. Although still fairly inefficient inhibitors of translation, the synthesized compounds represent promising chemical scaffolds that target the peptidyl transferase center of the ribosome and potentially are suitable for further exploration.

Atomic mutagenesis of stop codon nucleotides reveals the chemical prerequisites for release factor-mediated peptide release

Termination of protein synthesis is triggered by the recognition of a stop codon at the ribosomal A site and is mediated by class I release factors (RFs). Whereas in bacteria, RF1 and RF2 promote termination at UAA/UAG and UAA/UGA stop codons, respectively, eukaryotes only depend on one RF (eRF1) to initiate peptide release at all three stop codons. Based on several structural as well as biochemical studies, interactions between mRNA, tRNA, and rRNA have been proposed to be required for stop codon recognition. In this study, the influence of these interactions was investigated by using chemically modified stop codons. Single functional groups within stop codon nucleotides were substituted to weaken or completely eliminate specific interactions between the respective mRNA and RFs. Our findings provide detailed insight into the recognition mode of bacterial and eukaryotic RFs, thereby revealing the chemical groups of nucleotides that define the identity of stop codons and provide the means to discriminate against noncognate stop codons or UGG sense codons.

Environmental and genetic modulation of the phenotypic expression of antibiotic resistance

Antibiotic resistance can be acquired by mutation or horizontal transfer of a resistance gene, and generally an acquired mechanism results in a predictable increase in phenotypic resistance. However, recent findings suggest that the environment and/or the genetic context can modify the phenotypic expression of specific resistance genes/mutations. An important implication from these findings is that a given genotype does not always result in the expected phenotype. This dissociation of genotype and phenotype has important consequences for clinical bacteriology and for our ability to predict resistance phenotypes from genetics and DNA sequences. A related problem concerns the degree to which the genes/mutations currently identified in vitro can fully explain the in vivo resistance phenotype, or whether there is a significant additional amount of presently unknown mutations/genes (genetic ‘dark matter’) that could contribute to resistance in clinical isolates. Finally, a very important question is whether/how we can identify the genetic features that contribute to making a successful pathogen, and predict why some resistant clones are very successful and spread globally? In this review, we describe different environmental and genetic factors that influence phenotypic expression of antibiotic resistance genes/mutations and how this information is needed to understand why particular resistant clones spread worldwide and to what extent we can use DNA sequences to predict evolutionary success.

Structural insights of lincosamides targeting the ribosome of Staphylococcus aureus

Antimicrobial resistance within a wide range of pathogenic bacteria is an increasingly serious threat to global public health. Among these pathogenic bacteria are the highly resistant, versatile and possibly aggressive bacteria, Staphylococcus aureus. Lincosamide antibiotics were proved to be effective against this pathogen. This small, albeit important group of antibiotics is mostly active against Gram-positive bacteria, but also used against selected Gram-negative anaerobes and protozoa. S. aureus resistance to lincosamides can be acquired by modifications and/or mutations in the rRNA and rProteins. Here, we present the crystal structures of the large ribosomal subunit of S. aureus in complex with the lincosamides lincomycin and RB02, a novel semisynthetic derivative and discuss the biochemical aspects of the in vitro potency of various lincosamides. These results allow better understanding of the drugs selectivity as well as the importance of the various chemical moieties of the drug for binding and inhibition.

Critical 23S rRNA interactions for macrolide-dependent ribosome stalling on the ErmCL nascent peptide chain

The nascent peptide exit tunnel has recently been identified as a functional region of ribosomes contributing to translation regulation and co-translational protein folding. Inducible expression of the erm resistance genes depends on ribosome stalling at specific codons of an upstream open reading frame in the presence of an exit tunnel-bound macrolide antibiotic. The molecular basis for this translation arrest is still not fully understood. Here, we used a nucleotide analog interference approach to unravel important functional groups on 23S rRNA residues in the ribosomal exit tunnel for ribosome stalling on the ErmC leader peptide. By replacing single nucleobase functional groups or even single atoms we were able to demonstrate the importance of A2062, A2503 and U2586 for drug-dependent ribosome stalling. Our data show that the universally conserved A2062 and A2503 are capable of forming a non-Watson–Crick base pair that is critical for sensing and transmitting the stalling signal from the exit tunnel back to the peptidyl transferase center of the ribosome. The nucleobases of A2062, A2503 as well as U2586 do not contribute significantly to the overall mechanism of protein biosynthesis, yet their elaborate role for co-translational monitoring of nascent peptide chains inside the exit tunnel can explain their evolutionary conservation.

Identification of Arabidopsis thaliana upstream open reading frames encoding peptide sequences that cause ribosomal arrest

Specific sequences of certain nascent peptides cause programmed ribosomal arrest during mRNA translation to control gene expression. In eukaryotes, most known regulatory arrest peptides are encoded by upstream open reading frames (uORFs) present in the 5′-untranslated region of mRNAs. However, to date, a limited number of eukaryotic uORFs encoding arrest peptides have been reported. Here, we searched for arrest peptide-encoding uORFs among Arabidopsis thaliana uORFs with evolutionarily conserved peptide sequences. Analysis of in vitro translation products of 22 conserved uORFs identified three novel uORFs causing ribosomal arrest in a peptide sequence-dependent manner. Stop codon-scanning mutagenesis, in which the effect of changing the uORF stop codon position on the ribosomal arrest was examined, and toeprint analysis revealed that two of the three uORFs cause ribosomal arrest during translation elongation, whereas the other one causes ribosomal arrest during translation termination. Transient expression assays showed that the newly identified arrest-causing uORFs exerted a strong sequence-dependent repressive effect on the expression of the downstream reporter gene in A. thaliana protoplasts. These results suggest that the peptide sequences of the three uORFs identified in this study cause ribosomal arrest in the uORFs, thereby repressing the expression of proteins encoded by the main ORFs.

Structural insights into substrate selectivity of ribosomal RNA methyltransferase RlmCD

RlmCD has recently been identified as the S-adenosyl methionine (SAM)-dependent methyltransferase responsible for the formation of m⁵U at U747 and U1939 of 23S ribosomal RNA in Streptococcus pneumoniae. In this research, we determine the high-resolution crystal structures of apo-form RlmCD and its complex with SAH. Using an in-vitro methyltransferase assay, we reveal the crucial residues for its catalytic functions. Furthermore, structural comparison between RlmCD and its structural homologue RumA, which only catalyzes the m⁵U1939 in Escherichia coli, implicates that a unique long linker in the central domain of RlmCD is the key factor in determining its substrate selectivity. Its significance in the enzyme activity of RlmCD is further confirmed by in-vitro methyltransferase assay.

Co-produced natural ketolides methymycin and pikromycin inhibit bacterial growth by preventing synthesis of a limited number of proteins

Antibiotics methymycin (MTM) and pikromycin (PKM), co-produced by Streptomyces venezuelae, represent minimalist macrolide protein synthesis inhibitors. Unlike other macrolides, which carry several side chains, a single desosamine sugar is attached to the macrolactone ring of MTM and PKM. In addition, the macrolactone scaffold of MTM is smaller than in other macrolides. The unusual structure of MTM and PKM and their simultaneous secretion by S. venezuelae bring about the possibility that two compounds would bind to distinct ribosomal sites. However, by combining genetic, biochemical and crystallographic studies, we demonstrate that MTM and PKM inhibit translation by binding to overlapping sites in the ribosomal exit tunnel. Strikingly, while MTM and PKM readily arrest the growth of bacteria, ∼40% of cellular proteins continue to be synthesized even at saturating concentrations of the drugs. Gel electrophoretic analysis shows that compared to other ribosomal antibiotics, MTM and PKM prevent synthesis of a smaller number of cellular polypeptides illustrating a unique mode of action of these antibiotics.

Identification of potential allosteric communication pathways between functional sites of the bacterial ribosome by graph and elastic network models

BACKGROUND

Accumulated evidence indicates that bacterial ribosome employs allostery throughout its structure for protein synthesis. The nature of the allosteric communication between remote functional sites remains unclear, but the contact topology and dynamics of residues may play role in transmission of a perturbation to distant sites.

METHODS/RESULTS

We employ two computationally efficient approaches - graph and elastic network modeling to gain insights about the allosteric communication in ribosome. Using graph representation of the structure, we perform k-shortest pathways analysis between peptidyl transferase center-ribosomal tunnel, decoding center-peptidyl transferase center - previously reported functional sites having allosteric communication. Detailed analysis on intact structures points to common and alternative shortest pathways preferred by different states of translation. All shortest pathways capture drug target sites and allosterically important regions. Elastic network model further reveals that residues along all pathways have the ability of quickly establishing pair-wise communication and to help the propagation of a perturbation in long-ranges during functional motions of the complex.

CONCLUSIONS

Contact topology and inherent dynamics of ribosome configure potential communication pathways between functional sites in different translation states. Inter-subunit bridges B2a, B3 and P-tRNA come forward for their high potential in assisting allostery during translation. Especially B3 emerges as a potential druggable site.

GENERAL SIGNIFICANCE

This study indicates that the ribosome topology forms a basis for allosteric communication, which can be disrupted by novel drugs to kill drug-resistant bacteria. Our computationally efficient approach not only overlaps with experimental evidence on allosteric regulation in ribosome but also proposes new druggable sites.

The macrolide antibiotic renaissance

Macrolides represent a large family of protein synthesis inhibitors of great clinical interest due to their applicability to human medicine. Macrolides are composed of a macrocyclic lactone of different ring sizes, to which one or more deoxy-sugar or amino sugar residues are attached. Macrolides act as antibiotics by binding to bacterial 50S ribosomal subunit and interfering with protein synthesis. The high affinity of macrolides for bacterial ribosomes, together with the highly conserved structure of ribosomes across virtually all of the bacterial species, is consistent with their broad-spectrum activity. Since the discovery of the progenitor macrolide, erythromycin, in 1950, many derivatives have been synthesised, leading to compounds with better bioavailability and acid stability and improved pharmacokinetics. These efforts led to the second generation of macrolides, including well-known members such as azithromycin and clarithromycin. Subsequently, in order to address increasing antibiotic resistance, a third generation of macrolides displaying improved activity against many macrolide resistant strains was developed. However, these improvements were accompanied with serious side effects, leading to disappointment and causing many researchers to stop working on macrolide derivatives, assuming that this procedure had reached the end. In contrast, a recent published breakthrough introduced a new chemical platform for synthesis and discovery of a wide range of diverse macrolide antibiotics. This chemical synthesis revolution, in combination with reduction in the side effects, namely, 'Ketek effects', has led to a macrolide renaissance, increasing the hope for novel and safe therapeutic agents to combat serious human infectious diseases.

An antimicrobial peptide that inhibits translation by trapping release factors on the ribosome

Many antibiotics stop bacterial growth by inhibiting different steps of protein synthesis. However, no specific inhibitors of translation termination are known. Proline-rich antimicrobial peptides, a component of the antibacterial defense system of multicellular organisms, interfere with bacterial growth by inhibiting translation. Here we show that Api137, a derivative of the insect-produced antimicrobial peptide apidaecin, arrests terminating ribosomes using a unique mechanism of action. Api137 binds to the Escherichia coli ribosome and traps release factors 1 or 2 subsequent to release of the nascent polypeptide chain. A high-resolution cryo-EM structure of the ribosome complexed with release factor 1 and Api137 reveals the molecular interactions that lead to release factor trapping. Api137-mediated depletion of the cellular pool of free release factors causes the majority of ribosomes to stall at stop codons prior to polypeptide release, thereby resulting in a global shutdown of translation termination.

The Ribosomal Protein uL22 Modulates the Shape of the Protein Exit Tunnel

Erythromycin is a clinically useful antibiotic that binds to an rRNA pocket in the ribosomal exit tunnel. Commonly, resistance to erythromycin is acquired by alterations of rRNA nucleotides that interact with the drug. Mutations in the β hairpin of ribosomal protein uL22, which is rather distal to the erythromycin binding site, also generate resistance to the antibiotic. We have determined the crystal structure of the large ribosomal subunit from Deinococcus radiodurans with a three amino acid insertion within the β hairpin of uL22 that renders resistance to erythromycin. The structure reveals a shift of the β hairpin of the mutated uL22 toward the interior of the exit tunnel, triggering a cascade of structural alterations of rRNA nucleotides that propagate to the erythromycin binding pocket. Our findings support recent studies showing that the interactions between uL22 and specific sequences within nascent chains trigger conformational rearrangements in the exit tunnel.

High-Efficiency "-1" and "-2" Ribosomal Frameshiftings Revealed by Force Spectroscopy

Ribosomal frameshifting is a rare but ubiquitous process that is being studied extensively. Meanwhile, frameshifting motifs without any secondary mRNA structures were identified but rarely studied experimentally. We report unambiguous observation of highly efficient “–1” and “–2” frameshiftings on a GA₇G slippery mRNA without the downstream secondary structure, using force-induced remnant magnetization spectroscopy combined with unique probing schemes. The result represents the first experimental evidence of multiple frameshifting steps. It is also one of the rare reports of the “–2” frameshifting. Our assay removed the ambiguity of transcriptional slippage involvement in other frameshifting assays. Two significant insights for the frameshifting mechanism were revealed. First, EF-G·GTP is indispensable to frameshifting. Although EFG·GDPCP has been shown to prompt translocation before, we found that it could not induce frameshifting. This implies that the GTP hydrolysis is responsible for the codon–anticodon re-pairing in frameshifting, which corroborates our previous mechanical force measurement of EF-G·GTP. Second, translation in all three reading frames of the slippery sequence can be induced by the corresponding in-frame aminoacyl tRNAs. Although A-site tRNA is known to affect the partition between “0” and “–1” frameshifting, it has not been reported that all three reading frames can be translated by their corresponding tRNAs. The in vitro results were confirmed by toe-printing assay and protein sequencing.

The Berkeleylactones, Antibiotic Macrolides from Fungal Coculture

A carefully timed coculture fermentation of Penicillium fuscum and P. camembertii/clavigerum yielded eight new 16-membered-ring macrolides, berkeleylactones A-H (1, 4, 6-9, 12, 13), as well as the known antibiotic macrolide A26771B (5), patulin, and citrinin. There was no evidence of the production of the berkeleylactones or A26771B (5) by either fungus when grown as axenic cultures. The structures were deduced from analyses of spectral data, and the absolute configurations of compounds 1 and 9 were determined by single-crystal X-ray crystallography. Berkeleylactone A (1) exhibited the most potent antimicrobial activity of the macrolide series, with low micromolar activity (MIC = 1-2 μg/mL) against four MRSA strains, as well as Bacillus anthracis, Streptococcus pyogenes, Candida albicans, and Candida glabrata. Mode of action studies have shown that, unlike other macrolide antibiotics, berkeleylactone A (1) does not inhibit protein synthesis nor target the ribosome, which suggests a novel mode of action for its antibiotic activity.

Regulation of antibiotic-resistance by non-coding RNAs in bacteria

Antibiotic resistance genes are commonly regulated by sophisticated mechanisms that activate gene expression in response to antibiotic exposure. Growing evidence suggest that cis-acting non-coding RNAs play a major role in regulating the expression of many resistance genes, specifically those which counteract the effects of translation-inhibiting antibiotics. These ncRNAs reside in the 5'UTR of the regulated gene, and sense the presence of the antibiotics by recruiting translating ribosomes onto short upstream open reading frames (uORFs) embedded in the ncRNA. In the presence of translation-inhibiting antibiotics ribosomes arrest over the uORF, altering the RNA structure of the regulator and switching the expression of the resistance gene to 'ON'. The specificity of these riboregulators is tuned to sense-specific classes of antibiotics based on the length and composition of the respective uORF. Here we review recent work describing new types of antibiotic-sensing RNA-based regulators and elucidating the molecular mechanisms by which they function to control antibiotic resistance in bacteria.

Sucrose sensing through nascent peptide-meditated ribosome stalling at the stop codon of Arabidopsis bZIP11 uORF2

Arabidopsis bZIP11 is a transcription factor that modulates amino acid metabolism under high-sucrose conditions. Expression of bZIP11 is downregulated in a sucrose-dependent manner during translation. Previous in vivo studies have identified the second upstream open reading frame (uORF2) as an essential regulatory element for the sucrose-dependent translational repression of bZIP11. However, it remains unclear how uORF2 represses bZIP11 expression under high-sucrose conditions. Through biochemical experiments using cell-free translation systems, we report on sucrose-mediated ribosome stalling at the stop codon of uORF2. The C-terminal 10 amino acids (29-SFSVxFLxxLYYV-41) of uORF2 are important for ribosome stalling. Our results demonstrate that uORF2 encodes a regulatory nascent peptide that functions to sense intracellular sucrose abundance. This is the first biochemical identification of the intracellular sucrose sensor.

Dual effect of chloramphenicol peptides on ribosome inhibition

Chloramphenicol peptides were recently established as useful tools for probing nascent polypeptide chain interaction with the ribosome, either biochemically, or structurally. Here, we present a new 10mer chloramphenicol peptide, which exerts a dual inhibition effect on the ribosome function affecting two distinct areas of the ribosome, namely the peptidyl transferase center and the polypeptide exit tunnel. According to our data, the chloramphenicol peptide bound on the chloramphenicol binding site inhibits the formation of both acetyl-phenylalanine-puromycin and acetyl-lysine-puromycin, showing, however, a decreased peptidyl transferase inhibition compared to chloramphenicol-mediated inhibition per se. Additionally, we found that the same compound is a strong inhibitor of green fluorescent protein synthesis in a coupled in vitro transcription-translation assay as well as a potent inhibitor of lysine polymerization in a poly(A)-programmed ribosome, showing that an additional inhibitory effect may exist. Since chemical protection data supported the interaction of the antibiotic with bases A2058 and A2059 near the entrance of the tunnel, we concluded that the extra inhibition effect on the synthesis of longer peptides is coming from interactions of the peptide moiety of the drug with residues comprising the ribosomal tunnel, and by filling up the tunnel and blocking nascent chain progression through the restricted tunnel. Therefore, the dual interaction of the chloramphenicol peptide with the ribosome increases its inhibitory effect and opens a new window for improving the antimicrobial potency of classical antibiotics or designing new ones.

Translational Repression of a Splice Variant of Cynomolgus Macaque CXCL1L by Its C-Terminal Sequence

We previously isolated a cDNA clone from cynomolgus macaque encoding a novel CXC chemokine that we termed CXCL1L from its close similarity to CXCL1. However, the cDNA consisted of 3 exons instead of 4 exons that were typically seen in other CXC chemokines. Here, we isolated a cDNA encoding the full-length variant of CXCL1L that we termed CXCL1Lβ. CXCL1Lβ is 50 amino acids longer than the original CXCL1L, which we now term CXCL1Lα. The CXCL1Lβ mRNA is much more abundantly expressed in the cynomolgus macaque tissues than CXCL1Lα mRNA. However, CXCL1Lβ protein was poorly produced by transfected cells compared with that of CXCL1Lα. When the coding region of the fourth exon was fused to the C-terminus of CXCL1 or even to a nonsecretory protein firefly luciferase, the fused proteins were also barely produced, although the mRNAs were abundantly expressed. The polysome profiling analysis suggested that the inhibition was mainly at the translational level. Furthermore, we demonstrated that the C-terminal 5 amino acids of CXCL1Lβ were critical for the translational repression. The present study, thus, reveals a unique translational regulation controlling the production of a splicing variant of CXCL1L. Since the CXCL1L gene is functional only in the Old World monkeys, we also discuss possible reasons for the conservation of the active CXCL1L gene in these monkeys during the primate evolution.

The Expression of Antibiotic Resistance Methyltransferase Correlates with mRNA Stability Independently of Ribosome Stalling

Members of the Erm methyltransferase family modify 23S rRNA of the bacterial ribosome and render cross-resistance to macrolides and multiple distantly related antibiotics. Previous studies have shown that the expression of erm is activated when a macrolide-bound ribosome stalls the translation of the leader peptide preceding the cotranscribed erm. Ribosome stalling is thought to destabilize the inhibitory stem-loop mRNA structure and exposes the erm Shine-Dalgarno (SD) sequence for translational initiation. Paradoxically, mutations that abolish ribosome stalling are routinely found in hyper-resistant clinical isolates; however, the significance of the stalling-dead leader sequence is largely unknown. Here, we show that nonsense mutations in the Staphylococcus aureus ErmB leader peptide (ErmBL) lead to high basal and induced expression of downstream ErmB in the absence or presence of macrolide concomitantly with elevated ribosome methylation and resistance. The overexpression of ErmB is associated with the reduced turnover of the ermBL-ermB transcript, and the macrolide appears to mitigate mRNA cleavage at a site immediately downstream of the ermBL SD sequence. The stabilizing effect of antibiotics on mRNA is not limited to ermBL-ermB; cationic antibiotics representing a ribosome-stalling inducer and a noninducer increase the half-life of specific transcripts. These data unveil a new layer of ermB regulation and imply that ErmBL translation or ribosome stalling serves as a “tuner” to suppress aberrant production of ErmB because methylated ribosome may impose a fitness cost on the bacterium as a result of misregulated translation.

Context-specific inhibition of translation by ribosomal antibiotics targeting the peptidyl transferase center

The first broad-spectrum antibiotic chloramphenicol and one of the newest clinically important antibacterials, linezolid, inhibit protein synthesis by targeting the peptidyl transferase center of the bacterial ribosome. Because antibiotic binding should prevent the placement of aminoacyl-tRNA in the catalytic site, it is commonly assumed that these drugs are universal inhibitors of peptidyl transfer and should readily block the formation of every peptide bond. However, our in vitro experiments showed that chloramphenicol and linezolid stall ribosomes at specific mRNA locations. Treatment of bacterial cells with high concentrations of these antibiotics leads to preferential arrest of translation at defined sites, resulting in redistribution of the ribosomes on mRNA. Antibiotic-mediated inhibition of protein synthesis is most efficient when the nascent peptide in the ribosome carries an alanine residue and, to a lesser extent, serine or threonine in its penultimate position. In contrast, the inhibitory action of the drugs is counteracted by glycine when it is either at the nascent-chain C terminus or at the incoming aminoacyl-tRNA. The context-specific action of chloramphenicol illuminates the operation of the mechanism of inducible resistance that relies on programmed drug-induced translation arrest. In addition, our findings expose the functional interplay between the nascent chain and the peptidyl transferase center.

Resistance to Macrolide Antibiotics in Public Health Pathogens

Macrolide resistance mechanisms can be target-based with a change in a 23S ribosomal RNA (rRNA) residue or a mutation in ribosomal protein L4 or L22 affecting the ribosome's interaction with the antibiotic. Alternatively, mono- or dimethylation of A2058 in domain V of the 23S rRNA by an acquired rRNA methyltransferase, the product of an erm (erythromycin ribosome methylation) gene, can interfere with antibiotic binding. Acquired genes encoding efflux pumps, most predominantly mef(A) + msr(D) in pneumococci/streptococci and msr(A/B) in staphylococci, also mediate resistance. Drug-inactivating mechanisms include phosphorylation of the 2'-hydroxyl of the amino sugar found at position C5 by phosphotransferases and hydrolysis of the macrocyclic lactone by esterases. These acquired genes are regulated by either translation or transcription attenuation, largely because cells are less fit when these genes, especially the rRNA methyltransferases, are highly induced or constitutively expressed. The induction of gene expression is cleverly tied to the mechanism of action of macrolides, relying on antibiotic-bound ribosomes stalled at specific sequences of nascent polypeptides to promote transcription or translation of downstream sequences.

Antibiotics: where to throw the spanner in the ribosomal machinery?

The sheer molecular scale of the ribosome is intimidating to the traditional drug designer. By analyzing the ribosome as a series of 12 key target sites, this review seeks to make the ribosome ligand design process more manageable. Analysis of recently evaluated ribosomal structures, particularly those with bound antibiotics, indicates where the ligand target sites are located. This review employs current research data to map antibiotic binding across the ribosome. A number of neighboring ligand-binding sites are often contiguous and can be combined. Ligands that bind in close proximity can be combined into hybrid structures. The different ways antibiotics disrupt ribosomal function are also discussed. Antibiotics tend to inhibit conformational changes that are essential to the ribosomal mechanism.

Binding of Macrolide Antibiotics Leads to Ribosomal Selection against Specific Substrates Based on Their Charge and Size

Macrolide antibiotic binding to the ribosome inhibits catalysis of peptide bond formation between specific donor and acceptor substrates. Why particular reactions are problematic for the macrolide-bound ribosome remains unclear. Using comprehensive mutational analysis and biochemical experiments with synthetic substrate analogs, we find that the positive charge of these specific residues and the length of their side chains underlie inefficient peptide bond formation in the macrolide-bound ribosome. Even in the absence of antibiotic, peptide bond formation between these particular donors and acceptors is rather inefficient, suggesting that macrolides magnify a problem present for intrinsically difficult substrates. Our findings emphasize the existence of functional interactions between the nascent protein and the catalytic site of the ribosomal peptidyl transferase center.

A combined cryo-EM and molecular dynamics approach reveals the mechanism of ErmBL-mediated translation arrest

Nascent polypeptides can induce ribosome stalling, regulating downstream genes. Stalling of ErmBL peptide translation in the presence of the macrolide antibiotic erythromycin leads to resistance in Streptococcus sanguis. To reveal this stalling mechanism we obtained 3.6-Å-resolution cryo-EM structures of ErmBL-stalled ribosomes with erythromycin. The nascent peptide adopts an unusual conformation with the C-terminal Asp10 side chain in a previously unseen rotated position. Together with molecular dynamics simulations, the structures indicate that peptide-bond formation is inhibited by displacement of the peptidyl-tRNA A76 ribose from its canonical position, and by non-productive interactions of the A-tRNA Lys11 side chain with the A-site crevice. These two effects combine to perturb peptide-bond formation by increasing the distance between the attacking Lys11 amine and the Asp10 carbonyl carbon. The interplay between drug, peptide and ribosome uncovered here also provides insight into the fundamental mechanism of peptide-bond formation.

The molecular choreography of protein synthesis: translational control, regulation, and pathways

Translation of proteins by the ribosome regulates gene expression, with recent results underscoring the importance of translational control. Misregulation of translation underlies many diseases, including cancer and many genetic diseases. Decades of biochemical and structural studies have delineated many of the mechanistic details in prokaryotic translation, and sketched the outlines of eukaryotic translation. However, translation may not proceed linearly through a single mechanistic pathway, but likely involves multiple pathways and branchpoints. The stochastic nature of biological processes would allow different pathways to occur during translation that are biased by the interaction of the ribosome with other translation factors, with many of the steps kinetically controlled. These multiple pathways and branchpoints are potential regulatory nexus, allowing gene expression to be tuned at the translational level. As research focus shifts toward eukaryotic translation, certain themes will be echoed from studies on prokaryotic translation. This review provides a general overview of the dynamic data related to prokaryotic and eukaryotic translation, in particular recent findings with single-molecule methods, complemented by biochemical, kinetic, and structural findings. We will underscore the importance of viewing the process through the viewpoints of regulation, translational control, and heterogeneous pathways.

The stringent factor RelA adopts an open conformation on the ribosome to stimulate ppGpp synthesis

Under stress conditions, such as nutrient starvation, deacylated tRNAs bound within the ribosomal A-site are recognized by the stringent factor RelA, which converts ATP and GTP/GDP to (p)ppGpp. The signaling molecules (p)ppGpp globally rewire the cellular transcriptional program and general metabolism, leading to stress adaptation. Despite the additional importance of the stringent response for regulation of bacterial virulence, antibiotic resistance and persistence, structural insight into how the ribosome and deacylated-tRNA stimulate RelA-mediated (p)ppGpp has been lacking. Here, we present a cryo-EM structure of RelA in complex with the Escherichia coli 70S ribosome with an average resolution of 3.7 Å and local resolution of 4 to >10 Å for RelA. The structure reveals that RelA adopts a unique ‘open’ conformation, where the C-terminal domain (CTD) is intertwined around an A/T-like tRNA within the intersubunit cavity of the ribosome and the N-terminal domain (NTD) extends into the solvent. We propose that the open conformation of RelA on the ribosome relieves the autoinhibitory effect of the CTD on the NTD, thus leading to stimulation of (p)ppGpp synthesis by RelA.

Structures of proline-rich peptides bound to the ribosome reveal a common mechanism of protein synthesis inhibition

With bacterial resistance becoming a serious threat to global public health, antimicrobial peptides (AMPs) have become a promising area of focus in antibiotic research. AMPs are derived from a diverse range of species, from prokaryotes to humans, with a mechanism of action that often involves disruption of the bacterial cell membrane. Proline-rich antimicrobial peptides (PrAMPs) are instead actively transported inside the bacterial cell where they bind and inactivate specific targets. Recently, it was reported that some PrAMPs, such as Bac71 -35, oncocins and apidaecins, bind and inactivate the bacterial ribosome. Here we report the crystal structures of Bac71 -35, Pyrrhocoricin, Metalnikowin and two oncocin derivatives, bound to the Thermus thermophilus 70S ribosome. Each of the PrAMPs blocks the peptide exit tunnel of the ribosome by simultaneously occupying three well characterized antibiotic-binding sites and interferes with the initiation step of translation, thereby revealing a common mechanism of action used by these PrAMPs to inactivate protein synthesis. Our study expands the repertoire of PrAMPs and provides a framework for designing new-generation therapeutics.

Clarifying the Translational Pausing Landscape in Bacteria by Ribosome Profiling

The rate of protein synthesis varies according to the mRNA sequence in ways that affect gene expression. Global analysis of translational pausing is now possible with ribosome profiling. Here, we revisit an earlier report that Shine-Dalgarno sequences are the major determinant of translational pausing in bacteria. Using refinements in the profiling method as well as biochemical assays, we find that SD motifs have little (if any) effect on elongation rates. We argue that earlier evidence of pausing arose from two factors. First, in previous analyses, pauses at Gly codons were difficult to distinguish from pauses at SD motifs. Second, and more importantly, the initial study preferentially isolated long ribosome-protected mRNA fragments that are enriched in SD motifs. These findings clarify the landscape of translational pausing in bacteria as observed by ribosome profiling.

Nascent peptide assists the ribosome in recognizing chemically distinct small molecules

Regulation of gene expression in response to the changing environment is critical for cell survival. For instance, binding of macrolide antibiotics to the ribosome promote the translation arrest at the leader ORFs ermCL and ermBL necessary for inducing antibiotic resistance genes ermC and ermB. Cladinose-containing macrolides, like erythromycin (ERY), but not ketolides e.g., telithromycin (TEL), arrest translation of ermCL, while either ERY or TEL stall ermBL translation. How the ribosome distinguishes between chemically similar small molecules is unknown.

We show that single amino acid changes in the leader peptide switch the specificity of recognition of distinct molecules, triggering gene activation in response to only ERY, only TEL, to both antibiotics, or preventing stalling altogether. Thus, the ribosomal response to chemical signals can be modulated by minute changes in the nascent peptide, suggesting that protein sequences could have been optimized for rendering translation sensitive to environmental cues.

N6-Methylated Adenosine in RNA: From Bacteria to Humans

N6-methyladenosine (m(6)A) is ubiquitously present in the RNA of living organisms from Escherichia coli to humans. Methyltransferases that catalyze adenosine methylation are drastically different in specificity from modification of single residues in bacterial ribosomal or transfer RNA to modification of thousands of residues spread among eukaryotic mRNA. Interactions that are formed by m(6)A residues range from RNA-RNA tertiary contacts to RNA-protein recognition. Consequences of the modification loss might vary from nearly negligible to complete reprogramming of regulatory pathways and lethality. In this review, we summarized current knowledge on enzymes that introduce m(6)A modification, ways to detect m(6)A presence in RNA and the functional role of this modification everywhere it is present, from bacteria to humans.

Nucleotide modifications within bacterial messenger RNAs regulate their translation and are able to rewire the genetic code

Nucleotide modifications within RNA transcripts are found in every organism in all three domains of life. 6-methyladeonsine (m⁶A), 5-methylcytosine (m⁵C) and pseudouridine (Ψ) are highly abundant nucleotide modifications in coding sequences of eukaryal mRNAs, while m⁵C and m⁶A modifications have also been discovered in archaeal and bacterial mRNAs. Employing in vitro translation assays, we systematically investigated the influence of nucleotide modifications on translation. We introduced m⁵C, m⁶A, Ψ or 2′-O-methylated nucleotides at each of the three positions within a codon of the bacterial ErmCL mRNA and analyzed their influence on translation. Depending on the respective nucleotide modification, as well as its position within a codon, protein synthesis remained either unaffected or was prematurely terminated at the modification site, resulting in reduced amounts of the full-length peptide. In the latter case, toeprint analysis of ribosomal complexes was consistent with stalling of translation at the modified codon. When multiple nucleotide modifications were introduced within one codon, an additive inhibitory effect on translation was observed. We also identified the m⁵C modification to alter the amino acid identity of the corresponding codon, when positioned at the second codon position. Our results suggest a novel mode of gene regulation by nucleotide modifications in bacterial mRNAs.

Blast from the Past: Reassessing Forgotten Translation Inhibitors, Antibiotic Selectivity, and Resistance Mechanisms to Aid Drug Development

Protein synthesis is a major target within the bacterial cell for antibiotics. Investigations into ribosome-targeting antibiotics have provided much needed functional and structural insight into their mechanism of action. However, the increasing prevalence of multi-drug-resistant bacteria has limited the utility of our current arsenal of clinically relevant antibiotics, highlighting the need for the development of new classes. Recent structural studies have characterized a number of antibiotics discovered decades ago that have unique chemical scaffolds and/or utilize novel modes of action to interact with the ribosome and inhibit translation. Additionally, structures of eukaryotic cytoplasmic and mitochondrial ribosomes have provided further structural insight into the basis for specificity and toxicity of antibiotics. Together with our increased understanding of bacterial resistance mechanisms, revisiting our treasure trove of "forgotten" antibiotics could pave the way for the next generation of antimicrobial agents.

Probing the Translation Dynamics of Ribosomes Using Zero-Mode Waveguides

In order to coordinate the complex biochemical and structural feat of converting triple-nucleotide codons into their corresponding amino acids, the ribosome must physically manipulate numerous macromolecules including the mRNA, tRNAs, and numerous translation factors. The ribosome choreographs binding, dissociation, physical movements, and structural rearrangements so that they synergistically harness the energy from biochemical processes, including numerous GTP hydrolysis steps and peptide bond formation. Due to the dynamic and complex nature of translation, the large cast of ligands involved, and the large number of possible configurations, tracking the global time evolution or dynamics of the ribosome complex in translation has proven to be challenging for bulk methods. Conventional single-molecule fluorescence experiments on the other hand require low concentrations of fluorescent ligands to reduce background noise. The significantly reduced bimolecular association rates under those conditions limit the number of steps that can be observed within the time window available to a fluorophore. The advent of zero-mode waveguide (ZMW) technology has allowed the study of translation at near-physiological concentrations of labeled ligands, moving single-molecule fluorescence microscopy beyond focused model systems into studying the global dynamics of translation in realistic setups. This chapter reviews the recent works using the ZMW technology to dissect the mechanism of translation initiation and elongation in prokaryotes, including complex processes such as translational stalling and frameshifting. Given the success of the technology, similarly complex biological processes could be studied in near-physiological conditions with the controllability of conventional in vitro experiments.

Complex Regulation Pathways of AmpC-Mediated β-Lactam Resistance in Enterobacter cloacae Complex

Enterobacter cloacae complex (ECC), an opportunistic pathogen causing numerous infections in hospitalized patients worldwide, is able to resist β-lactams mainly by producing the AmpC β-lactamase enzyme. AmpC expression is highly inducible in the presence of some β-lactams, but the underlying genetic regulation, which is intricately linked to peptidoglycan recycling, is still poorly understood. In this study, we constructed different mutant strains that were affected in genes encoding enzymes suspected to be involved in this pathway. As expected, the inactivation of ampC, ampR (which encodes the regulator protein of ampC), and ampG (encoding a permease) abolished β-lactam resistance. Reverse transcription-quantitative PCR (qRT-PCR) experiments combined with phenotypic studies showed that cefotaxime (at high concentrations) and cefoxitin induced the expression of ampC in different ways: one involving NagZ (a N-acetyl-β-D-glucosaminidase) and another independent of NagZ. Unlike the model established for Pseudomonas aeruginosa, inactivation of DacB (also known as PBP4) was not responsible for a constitutive ampC overexpression in ECC, whereas it caused AmpC-mediated high-level β-lactam resistance, suggesting a post-transcriptional regulation mechanism. Global transcriptomic analysis by transcriptome sequencing (RNA-seq) of a dacB deletion mutant confirmed these results. Lastly, analysis of 37 ECC clinical isolates showed that amino acid changes in the AmpD sequence were likely the most crucial event involved in the development of high-level β-lactam resistance in vivo as opposed to P. aeruginosa where dacB mutations have been commonly found. These findings bring new elements for a better understanding of β-lactam resistance in ECC, which is essential for the identification of novel potential drug targets.

Resistance to ketolide antibiotics by coordinated expression of rRNA methyltransferases in a bacterial producer of natural ketolides

Ketolides are promising new antimicrobials effective against a broad range of Gram-positive pathogens, in part because of the low propensity of these drugs to trigger the expression of resistance genes. A natural ketolide pikromycin and a related compound methymycin are produced by Streptomyces venezuelae strain ATCC 15439. The producer avoids the inhibitory effects of its own antibiotics by expressing two paralogous rRNA methylase genes pikR1 and pikR2 with seemingly redundant functions. We show here that the PikR1 and PikR2 enzymes mono- and dimethylate, respectively, the N6 amino group in 23S rRNA nucleotide A2058. PikR1 monomethylase is constitutively expressed; it confers low resistance at low fitness cost and is required for ketolide-induced activation of pikR2 to attain high-level resistance. The regulatory mechanism controlling pikR2 expression has been evolutionary optimized for preferential activation by ketolide antibiotics. The resistance genes and the induction mechanism remain fully functional when transferred to heterologous bacterial hosts. The anticipated wide use of ketolide antibiotics could promote horizontal transfer of these highly efficient resistance genes to pathogens. Taken together, these findings emphasized the need for surveillance of pikR1/pikR2-based bacterial resistance and the preemptive development of drugs that can remain effective against the ketolide-specific resistance mechanism.