Regulation of the macrolide resistance ABC-F translation factor MsrD.

Antibiotic resistance ABC-Fs (ARE ABC-Fs) are translation factors that provide resistance against clinically important ribosome-targeting antibiotics which are proliferating among pathogens. Here, we combine genetic and structural approaches to determine the regulation of streptococcal ARE ABC-F gene msrD in response to macrolide exposure. We show that binding of cladinose-containing macrolides to the ribosome prompts insertion of the leader peptide MsrDL into a crevice of the ribosomal exit tunnel, which is conserved throughout bacteria and eukaryotes. This leads to a local rearrangement of the 23 S rRNA that prevents peptide bond formation and accommodation of release factors. The stalled ribosome obstructs the formation of a Rho-independent terminator structure that prevents msrD transcriptional attenuation. Erythromycin induction of msrD expression via MsrDL, is suppressed by ectopic expression of mrsD, but not by mutants which do not provide antibiotic resistance, showing correlation between MsrD function in antibiotic resistance and its action on this stalled complex.

Tetracenomycin X sequesters peptidyl-tRNA during translation of QK motifs.

As antimicrobial resistance threatens our ability to treat common bacterial infections, new antibiotics with limited cross-resistance are urgently needed. In this regard, natural products that target the bacterial ribosome have the potential to be developed into potent drugs through structure-guided design, provided their mechanisms of action are well understood. Here we use inverse toeprinting coupled to next-generation sequencing to show that the aromatic polyketide tetracenomycin X primarily inhibits peptide bond formation between an incoming aminoacyl-tRNA and a terminal Gln-Lys (QK) motif in the nascent polypeptide. Using cryogenic electron microscopy, we reveal that translation inhibition at QK motifs occurs via an unusual mechanism involving sequestration of the 3' adenosine of peptidyl-tRNALys in the drug-occupied nascent polypeptide exit tunnel of the ribosome. Our study provides mechanistic insights into the mode of action of tetracenomycin X on the bacterial ribosome and suggests a path forward for the development of novel aromatic polyketide antibiotics.

Structural and mechanistic basis for translation inhibition by macrolide and ketolide antibiotics.

Macrolides and ketolides comprise a family of clinically important antibiotics that inhibit protein synthesis by binding within the exit tunnel of the bacterial ribosome. While these antibiotics are known to interrupt translation at specific sequence motifs, with ketolides predominantly stalling at Arg/Lys-X-Arg/Lys motifs and macrolides displaying a broader specificity, a structural basis for their context-specific action has been lacking. Here, we present structures of ribosomes arrested during the synthesis of an Arg-Leu-Arg sequence by the macrolide erythromycin (ERY) and the ketolide telithromycin (TEL). Together with deep mutagenesis and molecular dynamics simulations, the structures reveal how ERY and TEL interplay with the Arg-Leu-Arg motif to induce translational arrest and illuminate the basis for the less stringent sequence-specific action of ERY over TEL. Because programmed stalling at the Arg/Lys-X-Arg/Lys motifs is used to activate expression of antibiotic resistance genes, our study also provides important insights for future development of improved macrolide antibiotics.