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

Ekaterina Dzyubak,Mee-Ngan F Yap

doi:10.1128/aac.01806-16

Abstract

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.

Highlights

Members of the Erm methyltransferase family modify 23S rRNA of the bacterial ribosome and render cross-resistance to macrolides and multiple distantly related antibiotics
Consistent with relevant clinical studies, we found that cells bearing truncated ErmB leader peptide (ErmBL) remain highly resistant via a previously underappreciated mechanism by which the antibiotic promotes the stability of ermB mRNA
The molecular- and atomic-level descriptions of macrolide-induced ribosome stalling have been elucidated in great detail [12,13,14,15,16,17, 19, 58, 59, 77,78,79,80,81]; the relationship between ribosome stalling and the cellular levels of Erm methyltransferase has not been entirely consistent with clinical findings, wherein inducible constitutive resistance is commonly found in strains bearing ribosome-stalling-dead leader peptides

Summary

Introduction

Members of the Erm methyltransferase family modify 23S rRNA of the bacterial ribosome and render cross-resistance to macrolides and multiple distantly related antibiotics. 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. Previous studies on the ermCL-ermC operon have shown that ErmC methyltransferase translation is activated by macrolides when the antibiotic-bound ribosome stalls at a specific site in the ermCL leader peptide upstream of the cotranscribed ermC. Most naturally occurring mutations are more subtle and, in many cases, result in premature termination before translation reaches the ribosome-stalling site These leader peptide mutants, which are presumably defective in ribosome stalling, remain responsive to macrolides that further upregulate the downstream erm expression.

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Conclusion