Toxin-antitoxin system
Type III toxin-antitoxin systems consist of a small RNA that binds directly to the toxin protein and inhibits its activity.
[11] Toxin-antitoxin systems have several biotechnological applications, such as maintaining plasmids in cell lines, targets for antibiotics, and as positive selection vectors.
[11] In Vibrio cholerae, multiple type II toxin-antitoxin systems located in a super-integron were shown to prevent the loss of gene cassettes.
[18] mazEF, a toxin-antitoxin locus found in E. coli and other bacteria, was proposed to induce programmed cell death in response to starvation, specifically a lack of amino acids.
[32] When bacteria are challenged with antibiotics, a small and distinct subpopulation of cells is able to withstand the treatment by a phenomenon dubbed as "persistence" (not to be confused with resistance).
[33] Due to their bacteriostatic properties, type II toxin-antitoxin systems have previously been thought to be responsible for persistence, by switching a fraction of the bacterial population to a dormant state.
[35][36][37] Toxin-antitoxin systems have been used as examples of selfish DNA as part of the gene centered view of evolution.
For example, the ccdAB system encoded in the chromosome of E. coli O157:H7 has been shown to be under negative selection, albeit at a slow rate due to its addictive properties.
[39] Toxins of type I systems are small, hydrophobic proteins that confer toxicity by damaging cell membranes.
[1] Few intracellular targets of type I toxins have been identified, possibly due to the difficult nature of analysing proteins that are poisonous to their bacterial hosts.
[10] Also, the detection of small proteins has been challenging due to technical issues, a problem that remains to be solved with large-scale analysis.
In the case of the well-characterised hok/sok system, in addition to the hok toxin and sok antitoxin, there is a third gene, called mok.
[39] In this system a labile proteic antitoxin tightly binds and inhibits the activity of a stable toxin.
[54] The proteins are typically around 100 amino acids in length,[39] and exhibit toxicity in a number of ways: CcdB, for example, affects DNA replication by poisoning DNA gyrase[55] whereas toxins from the MazF family are endoribonucleases that cleave cellular mRNAs,[56][57] tRNAs [58][59] or rRNAs [60] at specific sequence motifs.
As explained by the "Translation-reponsive model",[63] the degree of expression is inversely proportional to the concentration of the repressive TA complex.
Type III toxin-antitoxin systems rely on direct interaction between a toxic protein and an RNA antitoxin.
[78] Type VII has been proposed to include systems hha/tomB, tglT/takA and hepT/mntA, all of which neutralise toxin activity by post-translational chemical modification of amino acid residues.
Due to incomplete complementarity between the creA guide and the creAT promoter, the Cas complex does not cleave the DNA, but instead remains at the site, where it blocks access by RNA polymerase, preventing expression of the creT toxin (a natural instance of CRISPRi).
When expressed, the creT RNA will sequester the rare arginine codon tRNAUCU, stalling translation and halting cell metabolism.
In an experiment examining the effectiveness of the hok/sok locus, it was found that segregational stability of an inserted plasmid expressing beta-galactosidase was increased by between 8 and 22 times compared to a control culture lacking a toxin-antitoxin system.
[84] The gene of interest is then targeted to recombine into the ccdB locus, inactivating the transcription of the toxic protein.
CcdB is found in recombinant bacterial genomes and an inactivated version of CcdA is inserted into a linearised plasmid vector.
[83] Toxin-antitoxin systems can cause cell suicide in certain conditions, such as a lack of a lab-specific growth medium they would not encounter outside of the controlled laboratory set-up.
