Toxin-Antitoxin Systems of Mycobacterium smegmatis Are Essential for Cell Survival

Rebekah Frampton,Raphael B.M Aggio,Silas G Villas-Bôas,Vickery L Arcus,Gregory M Cook

doi:10.1074/jbc.m111.286856

Rebekah Frampton, Raphael B.M Aggio + Show 3 more

Open Access

https://doi.org/10.1074/jbc.m111.286856

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Abstract

The role of chromosomal toxin-antitoxin (TA) modules in bacterial physiology remains enigmatic despite their abundance in the genomes of many bacteria. Mycobacterium smegmatis contains three putative TA systems, VapBC, MazEF, and Phd/Doc, and previous work from our group has shown VapBC to be a bona fide TA system. In this study, we show that MazEF and Phd/Doc are also TA systems that are constitutively expressed, transcribed as leaderless transcripts, and subject to autoregulation, and expression of the toxin component leads to growth inhibition that can be rescued by the cognate antitoxin. No phenotype was identified for deletions of the individual TA systems, but a triple deletion strain (ΔvapBC, mazEF, phd/doc), designated ΔTA(triple), exhibited a survival defect in complex growth medium demonstrating an essential role for these TA modules in mycobacterial survival. Transcriptomic analysis revealed no significant differences in gene expression between wild type and the ΔTA(triple) mutant under these conditions suggesting that the growth defect was not at a transcriptional level. Metabolomic analysis demonstrated that in response to starvation in complex medium, both the wild type and ΔTA(triple) mutant consumed a wide range of amino acids from the external milieu. Analysis of intracellular metabolites revealed a significant difference in the levels of branched-chain amino acids between the wild type and ΔTA(triple) mutant, which are proposed to play essential roles in monitoring the nutritional supply and physiological state of the cell and linking catabolic with anabolic reactions. Disruption of this balance in the ΔTA(triple) mutant may explain the survival defect in complex growth medium.

Highlights

Mycobacteria harbor a vast array of toxin-antitoxin modules, but their roles remain largely unknown
We show that MazEF and Phd/Doc are TA systems that are constitutively expressed, transcribed as leaderless transcripts, and subject to autoregulation, and expression of the toxin component leads to growth inhibition that can be rescued by the cognate antitoxin
We show that all three TA systems are bona fide TA modules, and deleting all three in M. smegmatis leads to a survival defect in complex growth medium

Summary

Introduction

Mycobacteria harbor a vast array of toxin-antitoxin modules, but their roles remain largely unknown. No phenotype was identified for deletions of the individual TA systems, but a triple deletion strain (⌬vapBC, mazEF, phd/doc), designated ⌬TAtriple, exhibited a survival defect in complex growth medium demonstrating an essential role for these TA modules in mycobacterial survival. Analysis of intracellular metabolites revealed a significant difference in the levels of branched-chain amino acids between the wild type and ⌬TAtriple mutant, which are proposed to play essential roles in monitoring the nutritional supply and physiological state of the cell and linking catabolic with anabolic reactions. Disruption of this balance in the ⌬TAtriple mutant may explain the survival defect in complex growth medium

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