Abstract
Background:
Mycobacteria harbor a vast array of toxin-antitoxin modules, but their roles remain largely unknown.
Results:
Deletion of all TA modules in
Mycobacterium smegmatis
caused a survival defect and alterations in amino acid metabolism.
Conclusion:
We demonstrate an essential role for TA modules in mycobacterial metabolism and survival.
Significance:
These results may explain the basis for 88 TA modules in
M. tuberculosis
where metabolism must be tightly controlled.
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 (Δ
vapBC, 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.