Toxin‐antitoxin systems: novel mechanisms of toxin activity and antitoxin inhibition

Abstract

Toxin‐antitoxin (TA) modules function as regulatory switches that allow bacteria to adapt to rapidly changing environments. Under conditions of unrestrained growth, these systems are repressed. However, under conditions of stress, these systems are activated, leading to growth arrest and dormancy. Their central role controlling bacterial growth has led to renewed efforts to understand the molecular basis of TA function in order to develop strategies to exploit these systems for societal benefit. The E. coli mqsR gene was originally discovered for its role in promoting biofilm formation and dormancy. During the past five years, we have used biochemistry and structural biology to show that mqsRA is a bona fide toxin‐antitoxin system, in which MqsR is ribonuclease (RNase) toxin, and MqsA is its cognate antitoxin. Unpublished results from our group now demonstrate that MqsR defines a new hybrid family of toxin RNases and that its MqsA antitoxin inhibits MqsR via a novel mechanism. Specifically, we are using NMR chemical shift mapping coupled with mutagenesis and biochemistry to identify the residues that mediate mRNA binding and cleavage and show that MqsR exhibits sequence specificity outside the primary GCU cleavage sequence. Furthermore, our work is revealing that MqsA inhibits MqsR activity via a completely novel mechanism for antitoxins; i.e. it potentially inhibits substrate binding by engaging loops outside the MqsR active site that are critical for the interaction with mRNA. Finally, we extending these studies to evolutionarily related families to elucidate the molecular basis of conservation and divergence of enzyme function in this family. Collectively, these studies are not only revealing that MqsRA defines a novel subfamily of TA systems but, more critically, also leading to the identification of the key molecular details that will allow these systems to be directed for societal benefit.Support or Funding InformationNSF‐CAREER award (MCB0952550) to RP.