Antimicrobials: Leaky Barrier Boosts Antibiotic Action

RESEARCH HIGHLIGHTS

Nature Reviews Microbiology | AOP, published online 26 November 2012; doi:10.1038/nrmicro2931

ANTIMICROBIALS Leaky barrier boosts antibiotic action The nascent polypeptide exit tun‑ Even after exposure to 100‑fold the the drug), the chimeric protein by- nel (NPET), which accommodates minimum inhibitory concentration passed erythromycin. This ability was the newly synthesized proteins as they of erythromycin, protein production preserved when several synonymous physiochemical make their way out of the bacterial continued at ~6% of the normal codon substitutions were introduced properties of ribosome, is the target of macrolide level, and strikingly, telithromycin in the hns segment, showing that it is antibiotics. It was generally assumed treatment permitted protein synthe the amino acid sequence of the nas‑ the N terminus that these drugs inhibit bacterial sis at ~25% of the normal level. cent peptide, rather than the mRNA determine growth by causing a global arrest in Two-dimensional gel electrophoresis sequence, that accounts for drug whether a protein synthesis; however, a new revealed that many of the synthesized evasion. Furthermore, the protein protein can study now reveals that macrolides proteins were drug specific, indicat‑ HspQ, which contains an N terminus permit translation of a distinct subset ing that the chemical structure of the resembling that of H‑NS, was also circumvent of proteins, and that this could be macrolide determined the spectrum capable of by-passing erythromycin. the macrolide even more detrimental to the cell. of proteins synthesized. Together, these data suggest that the barrier. Macrolides bind to a narrow So what features of the protein physiochemical properties of the region of the NPET and were previ‑ define its ability to by-pass the drug? N terminus determine whether a ously believed to block the passage Mass spectrometry of the synthesized protein can circumvent the macrolide of all proteins. However, recent proteins revealed that the small barrier. crystallographic studies implied DNA-binding protein H‑NS was However, not all proteins with an that there might be enough space capable of by-passing all the tested N terminus that by-passed the drug for a nascent peptide to squeeze macrolides. Because the amino managed to escape translational arrest. through the obstructed tunnel. To terminus of the protein makes the In vitro translation of full-size elonga‑ investigate this further, Kannan et al. initial contact with the drug during tion factor G was inhibited by telithro‑ monitored 35S‑methionine incorpora‑ translation, this region of H‑NS was mycin, resulting in the production of tion into Escherichia coli proteins studied in more detail. When the first a truncated protein. Thus, the drug- following treatment of the cells with 18 codons of the hns gene were fused bound NPET retains some protein the macrolide erythromycin or the to the 5ʹ end of the osmC gene (which selectivity even after the N terminus more potent ketolide, telithromycin. encodes a protein that is blocked by has circumvented the drug. These findings were not limited to E. coli, but were also reproduced in Staphylococcus aureus, suggesting that this is a general phenomenon. The observation that macrolides cause a selective rather than a global arrest of protein synthesis could be an important factor that influences the effectiveness of these drugs. In par‑ ticular, the finding that telithromycin, which is a more potent antimicrobial than erythromycin, permits a higher level of residual protein synthesis would suggest that imbalances in the proteome are more fatal than complete shut-down of protein synthesis. Christina Tobin Kåhrström

ORIGINAL RESEARCH PAPER Kannan, K. et al. Selective protein synthesis by ribosomes with a drug-obstructed exit tunnel. Cell 151, 508–520 (2012) GETTY

NATURE REVIEWS | MICROBIOLOGY VOLUME 11 | JANUARY 2013