The tetracycline resistance protein Tet(o) perturbs the conformation of the ribosomal decoding centre

Proline-Hinged α-Helical Peptides Sensitize Gram-Positive Antibiotics, Expanding Their Physicochemical Properties to Be Used as Gram-Negative Antibiotics

The outer membrane (OM) of Gram-negative bacteria is the most difficult obstacle for small-molecule antibiotics to reach their targets in the cytosol. The molecular features of Gram-negative antibiotics required for passing through the OM are that they should be positively charged rather than neutral, flat rather than globular, less flexible, or more increased amphiphilic moment. Because of these specific molecular characteristics, developing Gram-negative antibiotics is difficult. We focused on sensitizer peptides to facilitate the passage of hydrophobic Gram-positive antibiotics through the OM. We explored ways of improving the sensitizing ability of proline-hinged α-helical peptides by adjusting their length, hydrophobicity, and N-terminal groups. A novel peptide, 1403, improves the potentiation of rifampicin in vitro and in vivo and potentiates most Gram-positive antibiotics. The "sensitizer" approach is more plausible than those that rely on conventional drug discovery methods concerning drug development costs and the development of drug resistance.

Structural basis for HflXr-mediated antibiotic resistance in Listeria monocytogenes

HflX is a ubiquitous bacterial GTPase that splits and recycles stressed ribosomes. In addition to HflX, Listeria monocytogenes contains a second HflX homolog, HflXr. Unlike HflX, HflXr confers resistance to macrolide and lincosamide antibiotics by an experimentally unexplored mechanism. Here, we have determined cryo-EM structures of L. monocytogenes HflXr-50S and HflX-50S complexes as well as L. monocytogenes 70S ribosomes in the presence and absence of the lincosamide lincomycin. While the overall geometry of HflXr on the 50S subunit is similar to that of HflX, a loop within the N-terminal domain of HflXr, which is two amino acids longer than in HflX, reaches deeper into the peptidyltransferase center. Moreover, unlike HflX, the binding of HflXr induces conformational changes within adjacent rRNA nucleotides that would be incompatible with drug binding. These findings suggest that HflXr confers resistance using an allosteric ribosome protection mechanism, rather than by simply splitting and recycling antibiotic-stalled ribosomes.

In Vitro Activity of the Novel Tetracyclines, Tigecycline, Eravacycline, and Omadacycline, Against Moraxella catarrhalis

Background

Tigecycline, eravacycline, and omadacycline are recently developed tetracyclines. Susceptibility of microbes to these tetracyclines and their molecular mechanisms have not been well elucidated. We investigated the susceptibility of Moraxella catarrhalis to tigecycline, eravacycline, and omadacycline and its resistance mechanisms against these tetracyclines.

Methods

A total of 207 non-duplicate M. catarrhalis isolates were collected from different inpatients. The minimum inhibitory concentrations (MICs) of the tetracyclines were determined by broth microdilution. Tigecycline-, eravacycline-, or omadacycline-resistant isolates were induced under in vitro pressure. The tet genes and mutations in the 16S rRNA was detected by PCR and sequencing.

Results

Eravacycline had a lower MIC₅₀ (0.06 mg/L) than tigecycline (0.125 mg/L) or omadacycline (0.125 mg/L) against M. catarrhalis isolates. We found that 136 isolates (65.7%) had the tetB gene, and 15 (7.2%) isolates were positive for tetL; however, their presence was not correlated with high tigecycline, eravacycline, or omadacycline (≥1 mg/L) MICs. Compared with the initial MIC after 160 days of induction, the MICs of tigecycline or eravacycline against three M. catarrhalis isolates increased ≥eight-fold, while those of omadacycline against two M. catarrhalis isolates increased 64-fold. Mutations in the 16S rRNA genes (C1036T and/or G460A) were observed in omadacycline-induced resistant isolates, and increased RR (the genes encoding 16SrRNA (four copies, RR1-RR4) copy number of 16S rRNA genes with mutations was associated with increased resistance to omadacycline.

Conclusions

Tigecycline, eravacycline, and omadacycline exhibited robust antimicrobial effects against M. catarrhalis. Mutations in the 16S rRNA genes contributed to omadacycline resistance in M. catarrhalis.

Antibiotic Resistance ABC-F Proteins: Bringing Target Protection into the Limelight

Members of the ATP-binding cassette (ABC)-F protein subfamily collectively mediate resistance to a broader range of clinically important antibiotic classes than any other group of resistance proteins and are widespread in pathogenic bacteria. Following over 25 years' of controversy regarding the mechanism by which these proteins work, it has recently been established that they provide antibiotic resistance through the previously recognized but underappreciated phenomenon of target protection; they bind to the ribosome to effect the release of ribosome-targeted antibiotics, thereby rescuing the translation apparatus from antibiotic-mediated inhibition. Here we review the ABC-F resistance proteins with an emphasis on their mechanism of action, first exploring the history of the debate about how these proteins work and outlining our current state of knowledge and then considering key questions to be addressed in understanding the molecular detail of their function.

The conserved GTPase HflX is a ribosome splitting factor that binds to the E-site of the bacterial ribosome

Using a combination of biochemical, structural probing and rapid kinetics techniques we reveal for the first time that the universally conserved translational GTPase (trGTPase) HflX binds to the E-site of the 70S ribosome and that its GTPase activity is modulated by peptidyl transferase centre (PTC) and peptide exit tunnel (PET) binding antibiotics, suggesting a previously undescribed mode of action for these antibiotics. Our rapid kinetics studies reveal that HflX functions as a ribosome splitting factor that disassembles the 70S ribosomes into its subunits in a nucleotide dependent manner. Furthermore, our probing and hydrolysis studies show that the ribosome is able to activate trGTPases bound to its E-site. This is, to our knowledge, the first case in which the hydrolytic activity of a translational GTPase is not activated by the GTPase activating centre (GAC) in the ribosomal A-site. Furthermore, we provide evidence that the bound state of the PTC is able to regulate the GTPase activity of E-site bound HflX.

Altered Machinery of Protein Synthesis in Alzheimer's: From the Nucleolus to the Ribosome

Ribosomes and protein synthesis have been reported to be altered in the cerebral cortex at advanced stages of Alzheimer's disease (AD). Modifications in the hippocampus with disease progression have not been assessed. Sixty-seven cases including middle-aged (MA) and AD stages I-VI were analyzed. Nucleolar chaperones nucleolin, nucleophosmin and nucleoplasmin 3, and upstream binding transcription factor RNA polymerase I gene (UBTF) mRNAs are abnormally regulated and their protein levels reduced in AD. Histone modifications dimethylated histone H3K9 (H3K9me2) and acetylated histone H3K12 (H3K12ac) are decreased in CA1. Nuclear tau declines in CA1 and dentate gyrus (DG), and practically disappears in neurons with neurofibrillary tangles. Subunit 28 ribosomal RNA (28S rRNA) expression is altered in CA1 and DG in AD. Several genes encoding ribosomal proteins are abnormally regulated and protein levels of translation initiation factors eIF2α, eIF3η and eIF5, and elongation factor eEF2, are altered in the CA1 region in AD. These findings show alterations in the protein synthesis machinery in AD involving the nucleolus, nucleus and ribosomes in the hippocampus in AD some of them starting at first stages (I-II) preceding neuron loss. These changes may lie behind reduced numbers of dendritic branches and reduced synapses of CA1 and DG neurons which cause hippocampal atrophy.

Altered machinery of protein synthesis is region- and stage-dependent and is associated with α-synuclein oligomers in Parkinson's disease

INTRODUCTION

Parkinson's disease (PD) is characterized by the accumulation of abnormal α-synuclein in selected regions of the brain following a gradient of severity with disease progression. Whether this is accompanied by globally altered protein synthesis is poorly documented. The present study was carried out in PD stages 1-6 of Braak and middle-aged (MA) individuals without alterations in brain in the substantia nigra, frontal cortex area 8, angular gyrus, precuneus and putamen.

RESULTS

Reduced mRNA expression of nucleolar proteins nucleolin (NCL), nucleophosmin (NPM1), nucleoplasmin 3 (NPM3) and upstream binding transcription factor (UBF), decreased NPM1 but not NPM3 nucleolar protein immunostaining in remaining neurons; diminished 18S rRNA, 28S rRNA; reduced expression of several mRNAs encoding ribosomal protein (RP) subunits; and altered protein levels of initiation factor eIF3 and elongation factor eEF2 of protein synthesis was found in the substantia nigra in PD along with disease progression. Although many of these changes can be related to neuron loss in the substantia nigra, selective alteration of certain factors indicates variable degree of vulnerability of mRNAs, rRNAs and proteins in degenerating sustantia nigra. NPM1 mRNA and 18S rRNA was increased in the frontal cortex area 8 at stage 5-6; modifications were less marked and region-dependent in the angular gyrus and precuneus. Several RPs were abnormally regulated in the frontal cortex area 8 and precuneus, but only one RP in the angular gyrus, in PD. Altered levels of eIF3 and eIF1, and decrease eEF1A and eEF2 protein levels were observed in the frontal cortex in PD. No modifications were found in the putamen at any time of the study except transient modifications in 28S rRNA and only one RP mRNA at stages 5-6. These observations further indicate marked region-dependent and stage-dependent alterations in the cerebral cortex in PD. Altered solubility and α-synuclein oligomer formation, assessed in total homogenate fractions blotted with anti-α-synuclein oligomer-specific antibody, was demonstrated in the substantia nigra and frontal cortex, but not in the putamen, in PD. Dramatic increase in α-synuclein oligomers was also seen in fluorescent-activated cell sorter (FACS)-isolated nuclei in the frontal cortex in PD.

CONCLUSIONS

Altered machinery of protein synthesis is altered in the substantia nigra and cerebral cortex in PD being the frontal cortex area 8 more affected than the angular gyrus and precuneus; in contrast, pathways of protein synthesis are apparently preserved in the putamen. This is associated with the presence of α-synuclein oligomeric species in total homogenates; substantia nigra and frontal cortex are enriched, albeit with different band patterns, in α-synuclein oligomeric species, whereas α-synuclein oligomers are not detected in the putamen.

Cryo-EM structure of the tetracycline resistance protein TetM in complex with a translating ribosome at 3.9-Å resolution

Ribosome protection proteins (RPPs) confer resistance to tetracycline by binding to the ribosome and chasing the drug from its binding site. Current models for RPP action are derived from 7.2- to 16-Å resolution structures of RPPs bound to vacant or nontranslating ribosomes. Here we present a cryo-electron microscopy reconstruction of the RPP TetM in complex with a translating ribosome at 3.9-Å resolution. The structure reveals the contacts of TetM with the ribosome, including interaction between the conserved and functionally critical C-terminal extension of TetM with a unique splayed conformation of nucleotides A1492 and A1493 at the decoding center of the small subunit. The resolution enables us to unambiguously model the side chains of the amino acid residues comprising loop III in domain IV of TetM, revealing that the tyrosine residues Y506 and Y507 are not responsible for drug-release as suggested previously but rather for intrafactor contacts that appear to stabilize the conformation of loop III. Instead, Pro509 at the tip of loop III is located directly within the tetracycline binding site where it interacts with nucleotide C1054 of the 16S rRNA, such that RPP action uses Pro509, rather than Y506/Y507, to directly dislodge and release tetracycline from the ribosome.

Tetracycline antibiotics and resistance mechanisms

The ribosome and protein synthesis are major targets within the cell for inhibition by antibiotics, such as the tetracyclines. The tetracycline family of antibiotics represent a large and diverse group of compounds, ranging from the naturally produced chlortetracycline, introduced into medical usage in the 1940s, to second and third generation semi-synthetic derivatives of tetracycline, such as doxycycline, minocycline and more recently the glycylcycline tigecycline. Here we describe the mode of interaction of tetracyclines with the ribosome and mechanism of action of this class of antibiotics to inhibit translation. Additionally, we provide an overview of the diverse mechanisms by which bacteria obtain resistance to tetracyclines, ranging from efflux, drug modification, target mutation and the employment of specialized ribosome protection proteins.

Negamycin induces translational stalling and miscoding by binding to the small subunit head domain of the Escherichia coli ribosome

Negamycin is a natural product with broad-spectrum antibacterial activity and efficacy in animal models of infection. Although its precise mechanism of action has yet to be delineated, negamycin inhibits cellular protein synthesis and causes cell death. Here, we show that single point mutations within 16S rRNA that confer resistance to negamycin are in close proximity of the tetracycline binding site within helix 34 of the small subunit head domain. As expected from its direct interaction with this region of the ribosome, negamycin was shown to displace tetracycline. However, in contrast to tetracycline-class antibiotics, which serve to prevent cognate tRNA from entering the translating ribosome, single-molecule fluorescence resonance energy transfer investigations revealed that negamycin specifically stabilizes near-cognate ternary complexes within the A site during the normally transient initial selection process to promote miscoding. The crystal structure of the 70S ribosome in complex with negamycin, determined at 3.1 Å resolution, sheds light on this finding by showing that negamycin occupies a site that partially overlaps that of tetracycline-class antibiotics. Collectively, these data suggest that the small subunit head domain contributes to the decoding mechanism and that small-molecule binding to this domain may either prevent or promote tRNA entry by altering the initial selection mechanism after codon recognition and before GTPase activation.

Mechanism of tetracycline resistance by ribosomal protection protein Tet(O)

Tetracycline resistance protein Tet(O), which protects the bacterial ribosome from binding the antibiotic tetracycline, is a translational GTPase with significant similarity in both sequence and structure to the elongation factor EF-G. Here, we present an atomic model of the Tet(O)-bound 70S ribosome based on our cryo-electron microscopic reconstruction at 9.6 Å resolution. This atomic model allowed us to identify the Tet(O)-ribosome binding sites, which involve three characteristic loops in domain 4 of Tet(O). Replacements of the three-amino acid tips of these loops by a single glycine residue result in loss of Tet(O)-mediated tetracycline resistance. On the basis of these findings, the mechanism of Tet(O)-mediated tetracycline resistance can be explained in molecular detail.

Structural basis for TetM-mediated tetracycline resistance

Ribosome protection proteins (RPPs) confer tetracycline resistance by binding to the ribosome and chasing the drug from its binding site. The current model for the mechanism of action of RPPs proposes that drug release is indirect and achieved via conformational changes within the drug-binding site induced upon binding of the RPP to the ribosome. Here we report a cryo-EM structure of the RPP TetM in complex with the 70S ribosome at 7.2-Å resolution. The structure reveals the contacts of TetM with the ribosome, including interaction between the conserved and functionally critical C-terminal extension of TetM and the decoding center of the small subunit. Moreover, we observe direct interaction between domain IV of TetM and the tetracycline binding site and identify residues critical for conferring tetracycline resistance. A model is presented whereby TetM directly dislodges tetracycline to confer resistance.

Molecular basis for different levels of tet(M) expression in Streptococcus pneumoniae clinical isolates

Seventy-four unrelated clinical isolates of Streptococcus pneumoniae harboring the tet(M) gene were studied. Seven strains with low tetracycline (Tc) MICs (0.25 to 0.5 μg/ml) were found to harbor truncated tet(M) alleles that were inactivated by different frameshift mutations. In contrast, five strains bore deletions in the tet(M) promoter region, among which four displayed increased Tc MICs (16 to 64 μg/ml). The same promoter mutations were detected in Tc-resistant mutants selected in vitro from various susceptible strains. Sequence analysis revealed that these deletions might impede the formation of the transcriptional attenuator located immediately upstream of tet(M). Expression in Enterococcus faecalis of a tet(M) reporter gene transcribed from these promoter mutants conferred a level of Tc resistance similar to that observed in the parental S. pneumoniae strains. These results show that different levels of Tc susceptibility found in clinical isolates of S. pneumoniae can be explained by frameshift mutations within tet(M) and by alterations of the upstream transcriptional attenuator.

Mutations in 16S rRNA that suppress cold-sensitive initiation factor 1 affect ribosomal subunit association

A mutation in the infA gene encoding initiation factor 1 (IF1) gives rise to a cold-sensitive phenotype. An Escherichia coli strain with this mutation was used as a tool to select for second-site suppressors that compensate for the cold sensitivity and map specifically to rRNA. Several suppressor mutants with altered 16S rRNA that partially restore growth of an IF1 mutant strain in the cold were isolated and characterized. Suppressor mutations were found in helix (h)18, h32, h34 and h41 in 16S rRNA. These mutations are not clustered to any particular region in 16S rRNA and none overlap previously reported sites of interaction with IF1. While the isolated suppressors are structurally diverse, they are functionally related because all affect ribosomal subunit association in vivo. Furthermore, in vitro subunit-association experiments indicate that most of the suppressor mutations directly influence ribosomal subunit association even though none of these are confined to any of the known intersubunit bridges. These results are consistent with the model that IF1 is an rRNA chaperone that induces large-scale conformational changes in the small ribosomal subunit, and as a consequence modulates initiation of translation by affecting subunit association.

Environmental macrolide-lincosamide-streptogramin and tetracycline resistant bacteria

Bacteria can become resistant to antibiotics by mutation, transformation, and/or acquisition of new genes which are normally associated with mobile elements (plasmids, transposons, and integrons). Mobile elements are the main driving force in horizontal gene transfer between strains, species, and genera and are responsible for the rapid spread of particular elements throughout a bacterial community and between ecosystems. Today, antibiotic resistant bacteria are widely distributed throughout the world and have even been isolated from environments that are relatively untouched by human civilization. In this review macrolides, lincosamides, streptogramins, and tetracycline resistance genes and bacteria will be discussed with an emphasis on the resistance genes which are unique to environmental bacteria which are defined for this review as species and genera that are primarily found outside of humans and animals.

The A-Z of bacterial translation inhibitors

Protein synthesis is one of the major targets in the cell for antibiotics. This review endeavors to provide a comprehensive "post-ribosome structure" A-Z of the huge diversity of antibiotics that target the bacterial translation apparatus, with an emphasis on correlating the vast wealth of biochemical data with more recently available ribosome structures, in order to understand function. The binding site, mechanism of action, and modes of resistance for 26 different classes of protein synthesis inhibitors are presented, ranging from ABT-773 to Zyvox. In addition to improving our understanding of the process of translation, insight into the mechanism of action of antibiotics is essential to the development of novel and more effective antimicrobial agents to combat emerging bacterial resistance to many clinically-relevant drugs.

Fate and transport of antibiotic residues and antibiotic resistance genes following land application of manure waste

Antibiotics are used in animal livestock production for therapeutic treatment of disease and at subtherapeutic levels for growth promotion and improvement of feed efficiency. It is estimated that approximately 75% of antibiotics are not absorbed by animals and are excreted in waste. Antibiotic resistance selection occurs among gastrointestinal bacteria, which are also excreted in manure and stored in waste holding systems. Land application of animal waste is a common disposal method used in the United States and is a means for environmental entry of both antibiotics and genetic resistance determinants. Concerns for bacterial resistance gene selection and dissemination of resistance genes have prompted interest about the concentrations and biological activity of drug residues and break-down metabolites, and their fate and transport. Fecal bacteria can survive for weeks to months in the environment, depending on species and temperature, however, genetic elements can persist regardless of cell viability. Phylogenetic analyses indicate antibiotic resistance genes have evolved, although some genes have been maintained in bacteria before the modern antibiotic era. Quantitative measurements of drug residues and levels of resistance genes are needed, in addition to understanding the environmental mechanisms of genetic selection, gene acquisition, and the spatiotemporal dynamics of these resistance genes and their bacterial hosts. This review article discusses an accumulation of findings that address aspects of the fate, transport, and persistence of antibiotics and antibiotic resistance genes in natural environments, with emphasis on mechanisms pertaining to soil environments following land application of animal waste effluent.

The weird and wonderful world of bacterial ribosome regulation

In every organism, translation of the genetic information into functional proteins is performed on the ribosome. In Escherichia coli up to 40% of the cell's total energy turnover is channelled toward the ribosome and protein synthesis. Thus, elaborate networks of translation regulation pathways have evolved to modulate gene expression in response to growth rate and external factors, ranging from nutrient deprivation, to chemical (pH, ionic strength) and physical (temperature) fluctuations. Since the fundamental players involved in regulation of the different phases of translation have already been extensively reviewed elsewhere, this review focuses on lesser known and characterized factors that regulate the ribosome, ranging from processing, modification and assembly factors, unusual initiation and elongation factors, to a variety of stress response proteins.

Evolution and ecology of antibiotic resistance genes

A new perspective on the topic of antibiotic resistance is beginning to emerge based on a broader evolutionary and ecological understanding rather than from the traditional boundaries of clinical research of antibiotic-resistant bacterial pathogens. Phylogenetic insights into the evolution and diversity of several antibiotic resistance genes suggest that at least some of these genes have a long evolutionary history of diversification that began well before the 'antibiotic era'. Besides, there is no indication that lateral gene transfer from antibiotic-producing bacteria has played any significant role in shaping the pool of antibiotic resistance genes in clinically relevant and commensal bacteria. Most likely, the primary antibiotic resistance gene pool originated and diversified within the environmental bacterial communities, from which the genes were mobilized and penetrated into taxonomically and ecologically distant bacterial populations, including pathogens. Dissemination and penetration of antibiotic resistance genes from antibiotic producers were less significant and essentially limited to other high G+C bacteria. Besides direct selection by antibiotics, there is a number of other factors that may contribute to dissemination and maintenance of antibiotic resistance genes in bacterial populations.

Update on acquired tetracycline resistance genes

This mini-review summarizes the changes in the field of bacterial acquired tetracycline resistance (tet) and oxytetracycline (otr) genes identified since the last major review in 2001. Thirty-eight acquired tetracycline resistant (Tc(r)) genes are known of which nine are new and include five genes coding for energy-dependent efflux proteins, two genes coding for ribosomal protection proteins, and two genes coding for tetracycline inactivating enzymes. The number of inactivating enzymes has increased from one to three, suggesting that work needs to be done to determine the role these enzymes play in bacterial resistance to tetracycline. In the same time period, 66 new genera have been identified which carry one or more of the previously described 29 Tc(r) genes. Included in the new genera is, for the first time, an obligate intracellular pathogen suggesting that this sheltered group of bacteria is capable of DNA exchange with non-obligate intracellular bacteria. The number of genera carrying ribosomal protection genes increased dramatically with the tet(M) gene now identified in 42 genera as compared with 24 and the tet(W) gene found in 17 new genera as compared to two genera in the last major review. New conjugative transposons, carrying different ribosomal protection tet genes, have been identified and an increase in the number of antibiotic resistance genes linked to tet genes has been found. Whether these new elements may help to spread the tet genes they carry to a wider bacterial host range is discussed.

Characterisation of viridans group streptococci with different levels of Tet(M)-mediated tetracycline resistance

Streptococcus oralis 264-3, Streptococcus mitis 254-1 and S. mitis 264-1, isolated from the oral cavities of two children were each found to carry the tet(M) gene but exhibited different degrees of reduced susceptibility to tetracycline (tetracycline MICs of 2, 8 and 64 mg/L, respectively). The aim of this study was to determine the molecular basis for the different levels of tetracycline resistance (Tc(R)) observed. Escherichia coli HB101 carrying the cloned tet(M) genes exhibited similar levels of tetracycline susceptibility to those observed in the parental streptococcal strains (MICs of 1, 16, and 64 mg/L for tet(M) genes from S. oralis 264-3, S. mitis 254-1 and S. mitis 264-1, respectively). DNA sequencing revealed that S. oralis 264-3 had a tet(M) gene highly homologous to tet(M) carried by Tn916 from Enterococcus faecalis (99.6% identity), while the intermediate- and high-level Tc(R) strains had tet(M) sequences that resembled the tet(M) gene of Tn5251 from Streptococcus pneumoniae (99.3% and 99.4% identity, respectively). No differences were observed in the upstream attenuator structure for each of the strains and differences in reduced tetracycline susceptibilities could be attributed to changes in the deduced amino acid sequences of the Tet(M) proteins.

Mechanism of Tet(O)-mediated tetracycline resistance

Tet(O) is an elongation factor-like protein which confers resistance to the protein synthesis inhibitor tetracycline by promoting the release of the drug from its inhibitory site on the ribosome. Here we investigated the interaction of Tet(O) with the elongating ribosome and show, using dimethyl sulfate (DMS) probing and binding assays, that it interacts preferentially with the post-translocational ribosome. Furthermore, using an XTP-dependent mutant of Tet(O), we demonstrated that Tet(O) induces conformational rearrangements within the ribosome which can be detected by EF-Tu, and manifested as a stimulation in the GTPase activity of this elongation factor. As such, these conformational changes probably involve the ribosomal GTPase-associated center and, accordingly, Tet(O) alters the DMS modification pattern of the L11 region. Additionally, tetracycline binding is associated with an E(a) of 58 kJ/mol. These results suggest a model where both Tet(O) and tetracycline induce a conformational change in functionally opposite directions and the Tet(O)-induced conformation persists after it has left the ribosome; this prevents rebinding of the drug while allowing productive A-site occupation by a ternary complex in the presence of tetracycline.