Antitoxin component of a type II toxin-antitoxin (TA) system. Labile antitoxin that binds to the MqsR mRNA interferase toxin and neutralizes its endoribonuclease activity. Overexpression prevents MqsR-mediated cessation of cell growth and inhibition of cell proliferation. Initially reported to act as a cotranscription factor with MqsA (PubMed:19690171, PubMed:20105222). Following further experiments, the MqsR-MqsA complex does not bind DNA and all reported data are actually due to a small fraction of free MqsA alone binding DNA. Addition of MqsR to a preformed MqsA-promoter DNA complex causes dissociation of the MqsA-DNA complex, probably causing derepression of MqsA-repressed transcripts (PubMed:23172222). MqsA binds to 2 palindromes in the promoter region of the mqsRA operon activating its transcription. Binds to other promoters, inducing mcbR and spy and repressing cspD among others (PubMed:20105222). Binds to and represses the rpoS promoter, the master stress regulator, resulting in decreased cyclic-di-GMP, reduced stress resistance, increased cell motility and decreased biofilm formation; in these experiments 5 TA systems are missing (lacks MazEF, RelEB, ChpB, YoeB-YefM, YafQ-DinJ) (PubMed:21516113). An earlier study showed overexpression alone increases biofilm formation, perhaps by repressing cspD; in these experiments the 5 TA systems are present (PubMed:20105222). Represses the csgD promoter. In the presence of stress, when this protein is degraded, the promoters it represses are derepressed, leading to biofilm formation (Probable). This TA system mediates cell growth during bile acid deoxycholate stress by degrading mRNA for probable deoxycholate-binding protein YgiS; bile acid detergents such as deoxycholate are important for host defense against bacterial growth in the gall bladder and duodenum (PubMed:25534751). Binds 1 Zn(2+) ion per subunit. The MqsR-MqsA complex is exceptionally thermostable with a Tm of 83.4 degress Celsius versus 48.1 degress Celsius for MqsR and 61.1 degress Celsius for MqsA. Homodimer. Crystallizes as a heterotetramer with MqsA, MqsR-MqsA(2)-MqsR (PubMed:20041169). Purifies as a probable heterohexamer of 2 MqsR dimers and 1 MqsA dimer (PubMed:19690171). Binds promoter DNA as a dimer (PubMed:21068382). When the 2 dissociate the MsqR mRNA interferase becomes active. Induced by amino acid starvation, glucose starvation and when translation is blocked. Induction is decreased in the absence of the Lon protease suggesting, by homology to other toxin-antitoxin systems, that Lon may degrade the MqsA antitoxin. Transcription is activated by MqsA (PubMed:20105222). It has been suggested that MqsA represses its own operon (PubMed:19690171). Not more induced in persister cells (PubMed:16768798). A member of the mqsRA operon. This operon induced by ectopic expression of toxins RelE, HicA and YafQ but not by MazF or HicA (PubMed:23432955). The Zn-binding N-terminal domain (residues 1-65) binds to the MqsR mRNA interferase toxin and makes contact with the DNA phosphate backbone, while the C-terminus (residues 70-131) binds the promoter in a sequence-specific manner. They are linked by a short flexible domain (PubMed:22789559). Degraded in the presence of oxidative stress, maybe by the Lon and/or ClpX proteases. Essential for growth, it cannot be disrupted (PubMed:16768798). A double mqsR-mqsA deletion leads to increased rpoS mRNA levels, resulting in increased cyclic-di-GMP levels, increasing stress resistance, increased biofilm formation (PubMed:21516113). The double mutant has increased metabolism and respiration in the presence of the bile acid deoxycholate and consequently grows less well. Decreases cell survival in the presence of 20% deoxycholate (PubMed:25534751).