Toxic component of a type II toxin-antitoxin (TA) system. Plays a significant role in the control of biofilm formation and induction of persister cells in the presence of antibiotics. An mRNA interferase which has been reported to be translation-independent (PubMed:19690171, PubMed:19943910, PubMed:23289863). It has also been reported to be translation-dependent (PubMed:20041169). Cleavage has been reported to occur on either side of G in the sequence GCU (PubMed:19690171). Also reported to cleave after C in GC(A/U) sequences (PubMed:19943910). There are only 14 genes in E.coli W3110 (and probably also MG1655) that do not have a GCU sequence and thus are resistant to the mRNA interferase activity; among these is the gene for toxin GhoT. Overexpression of MqsR causes cessation of cell growth and inhibits cell proliferation via inhibition of translation as well as increasing persister cell formation; these effects are overcome by concomitant or subsequent expression of antitoxin MqsA. Cross-talk can occur between different TA systems. Ectopic expression of this toxin induces transcription of the relBEF TA system operon with specific cleavage of the relBEF mRNA produced (PubMed:23432955). Regulates the expression of GhoT/GhoS, a type V TA system (PubMed:23289863). Persistence depends on toxin GhoT activity, which MqsR controls at the post-transcriptional level by selectively degrading the antitoxin ghoS segment of the ghoST mRNA (PubMed:23289863). Overexpression leads to a dramatic increase in tolerance to the antibiotic ofloxacin. 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). 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. Does not bind DNA in the presence or absence of MqsA (PubMed:23172222). 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. Might be a dimer (PubMed:19690171). Also reported to be a monomer (PubMed:23172222). Crystallizes as a heterotetramer with MqsA, MqsR-MqsA(2)-MqsR (PubMed:20041169). Purifies as a possible heterohexamer of 2 MqsR dimers and 1 MqsA dimer (PubMed:19690171). When the 2 dissociate the MsqR mRNA interferase becomes active. Induced by amino acid starvation, glucose starvation and when translation is blocked. Also induced by nalidixic acid, azolocillin and H(2)O(2) (PubMed:23289863). It has been suggested that MqsA represses its own operon (PubMed:19690171). 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). A member of the mqsRA operon. Most highly induced gene in persister cells (PubMed:16768798). Degrades its own transcript (PubMed:23172222). This operon induced by ectopic expression of toxins RelE, HicA and YafQ but not by MazF or HicA (PubMed:23432955). No loss of ability to form persister cells in MG1655, represses persister cell formation in BW25113. Deletion decreases biofilm formation in LB medium (PubMed:16352847). Deletion has also been shown to increase biofilm formation (PubMed:20105222). Deletion at 48h, in flow cells, leads to a reduction in biomass, substratum coverage and changes the biofilm architecture from a 54 micron thick film with microcolonies to one with nearly no biomass and only a few colonies remaining. Deletion abolishes motility. 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).