The Mycobacterium tuberculosis relBE toxin:antitoxin genes are stress-responsive modules that regulate growth through translation inhibition

Toxin-antitoxin (TA) genes are ubiquitous among bacteria and are associated with persistence and dormancy. Following exposure to unfavorable environmental stimuli, several species (Escherichia coli, Staphylococcus aureus, Myxococcus xanthus) employ toxin proteins such as RelE and MazF to downregulate growth or initiate cell death. Mycobacterium tuberculosis possesses three Rel TA modules (Rel (Mtb) ): RelBE (Mtb) , RelFG (Mtb) and RelJK (Mtb) (Rv1246c-Rv1247c, Rv2865-Rv2866, and Rv3357-Rv3358, respectively), which inhibit mycobacterial growth when the toxin gene (relE, relG, relK) is expressed independently of the antitoxin gene (relB, relF, relJ). In the present study, we examined the in vivo mechanism of the RelE (Mtb) toxin protein, the impact of RelE (Mtb) on M. tuberculosis physiology and the environmental conditions that regulate all three rel (Mtb) modules. RelE (Mtb) negatively impacts growth and the structural integrity of the mycobacterial envelope, generating cells with aberrant forms that are prone to extensive aggregation. At a time coincident with growth defects, RelE (Mtb) mediates mRNA degradation in vivo resulting in significant changes to the proteome. We establish that rel (Mtb) modules are stress responsive, as all three operons are transcriptionally activated following mycobacterial exposure to oxidative stress or nitrogen-limiting growth environments. Here we present evidence that the rel (Mtb) toxin:antitoxin family is stress-responsive and, through the degradation of mRNA, the RelE (Mtb) toxin influences the growth, proteome and morphology of mycobacterial cells.