Toxin–antitoxin (TA) systems play significant roles in the regulation of bacterial growth and persistence, and their functions usually depend on protein–protein interaction between their constituent TA proteins. However, the regulatory mechanisms of these systems, particularly their interaction with other cellular components, are still poorly understood. This study investigated cross-talk between the TA module RelJ/K (Rv3357/Rv3358) and the transcriptional regulator staphylococcal iron regulator repressor (SirR, Rv2788) from Mycobacterium tuberculosis. We characterized the physical interaction of SirR with both RelJ and RelK using bacterial two-hybrid, pull-down and co-immunoprecipitation assays. Similarly to RelK, SirR regulates the DNA-binding activity of RelJ and alleviates its inhibitory effect on the activity of the Rv3357p promoter. Furthermore, SirR may replace RelJ to alleviate the inhibitory effect of the toxin RelK on bacterial growth. Conversely, both RelJ and RelK competitively inhibit the interaction between SirR and their respective promoters. Thus, our results show that SirR interacts with a pair of toxin and antitoxin proteins, and exhibits antitoxin-like function to neutralize the toxin. These findings demonstrate a novel function of the SirR regulator of M. tuberculosis as well as a novel mechanism of regulation of TA systems.Structured digital abstract
The current study investigated cross-talks between the toxin-antitoxin module RelJ/K and the transcriptional repressor SirR from M. tuberculosis. Our results show that SirR exhibits antitoxin-like function to neutralize the toxin, while RelJ/K inhibit the DNA binding activity of SirR. These findings uncover a novel function of SirR as well as a novel regulatory mechanism of TA systems.