Evolutionarily distant bacterial species respond to zinc starvation by reprogramming their ribosome assembly, in which the constitutive ribosomal proteins with zinc-binding motifs CXXC (C+) are substituted with alternative zinc-free counterparts (C-) through a transcriptional de- repression mechanism involving the zinc uptake regulator, ZurB. The alternative ribosomes assembled with C- ribosomal proteins reduce the zinc requirement for cellular growth under zinc-limiting conditions. Mycobacterium tuberculosis (Mtb), the etiological agent for tuberculosis (TB), has four C+/C- ribosomal protein pairs. All four genes encoding C- proteins of the pair are organized in an operon and are co-expressed through a ZurB-repressible promoter. Co- expression implies simultaneous substitution of all four C+ proteins by C- paralogs, raising questions about the influence of the substitutions on ribosomal response to antibiotics. We have found that the alternative C- ribosomes in both Mtb and Mycobacterium smegmatis not only reduce the zinc requirement for cellular growth, but also confer tolerance to clinically relevant anti-TB ribosomal antibiotics. We further observed that Mtb express more alternative ribosomes during chronic infection than in early acute phase. We thus hypothesize that expression of alternative ribosomes are the primary reasons underlying the inefficacies of the clinically established ribosomal antibiotics against TB. To facilitate improved targeting of mycobacterial ribosomes we propose to: a) elucidate the mechanistic basis of differential responses of constitutive (C+) and alternative (C-) ribosomes to antibiotics (aim 1), b) identify strategies to target alternative (C-) ribosomes (aim 2) and c) determine the role of alternative ribosomes in pathogenesis of Mtb (aim 3). These studies will ultimately lead to new strategies to target both constitutive and alternative ribosomes in mycobacteria, and thus allow effective clearance of mycobacterial infections.

Public Health Relevance

The clinical incidence of drug resistance tuberculosis, generally referred to as Multi-Drug resistant (MDR-) or extensively drug resistant, XDR-TB, is rising at an alarming pace. New strategies to counter the drug resistant strains of the TB pathogen, Mycobacterium tuberculosis, is urgently needed. Antibiotics that target ribosomes are heavily used in treatment of drug resistant TB, although these antibiotics have limited efficacies. By investigating the role of alternative ribosomes in tolerance of mycobacteria to ribosomal antibiotics, this study will open up new possibilities of efficiently targeting mycobacterial ribosomes and thus control drug resistant TB.

National Institute of Health (NIH)
National Institute of Allergy and Infectious Diseases (NIAID)
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Drug Discovery and Mechanisms of Antimicrobial Resistance Study Section (DDR)
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Mendez, Susana
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Wadsworth Center
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Li, Yunlong; Sharma, Manjuli R; Koripella, Ravi K et al. (2018) Zinc depletion induces ribosome hibernation in mycobacteria. Proc Natl Acad Sci U S A 115:8191-8196
Yang, Yong; Richards, Jacob P; Gundrum, Jennifer et al. (2018) GlnR Activation Induces Peroxide Resistance in Mycobacterial Biofilms. Front Microbiol 9:1428
Yang, Yong; Thomas, Joseph; Li, Yunlong et al. (2017) Defining a temporal order of genetic requirements for development of mycobacterial biofilms. Mol Microbiol 105:794-809