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.