Small Toxic Proteins and the Antisense RNAs That Repress Them

Abstract

SUMMARY There has been a great expansion in the number of small regulatory RNAs identified in bacteria. Some of these small RNAs repress the synthesis of potentially toxic proteins. Generally the toxin proteins are hydrophobic and less than 60 amino acids in length, and the corresponding antitoxin small RNA genes are antisense to the toxin genes or share long stretches of complementarity with the target mRNAs. Given their short length, only a limited number of these type I toxin-antitoxin loci have been identified, but it is predicted that many remain to be found. Already their characterization has given insights into regulation by small RNAs, has suggested functions for the small toxic proteins at the cell membrane, and has led to practical applications for some of the type I toxin-antitoxin loci.

RETRACTED: Bacterial persistence by RNA endonucleases

“…Generally, prokaryotic TA loci code for two components: a toxin that inhibits cell growth and an antitoxin that counteracts the toxin. In type I TA loci, the antitoxins are small antisense RNAs that repress translation of the toxin genes (8,9), whereas in type II loci, the antitoxins are proteins that combine with and neutralize the toxins (10). Type III TA loci encode small RNA antitoxins that combine with and neutralize protein toxins (11).…”

The toxin-antitoxin systemtisB-istR1

“…For instance, E. coli K12 encodes four ldr/rdl loci, five ibs/sib loci, and a locus denoted shoB/ ohsC. [40][41][42] Bioinformatics suggests that they, and some additional type I systems, are abundantly present in other bacteria. 10 Their toxins, in most cases, are small hydrophobic proteins …”

“…Whole genome expression studies in strains expressing four of these small toxins, IbsC, ShoB, LdrD and TisB, revealed changes in the expression of a shared set of genes, but also indicated individual toxin-specific effects. 40 Another interesting type I system is symE/symR. The SOS-induced SymE toxin is 113 amino acids long, not hydrophobic, exhibits structural similarity to the antitoxin MazE (of mazE/mazF 43 ), and may induce RNA degradation.…”

“…Processing that are known, or suspected, to affect membrane integrity. 40 Gene organization is hok/sok-rather than tisB/istR1-like in that the transcription units of toxin and antitoxin overlap, i.e., they represent cis-encoded systems (except: shoB/ohsC). Whole genome expression studies in strains expressing four of these small toxins, IbsC, ShoB, LdrD and TisB, revealed changes in the expression of a shared set of genes, but also indicated individual toxin-specific effects.…”

Cells need safety valves

“…In the present situation the trade-off could be that the influx/modification/efflux system we have described produces a form of addiction: indeed, it has some abstract properties similar to those of the restriction/ modification systems, or the toxin/antitoxin systems. (35,36) It will be interesting to explore whether this is used in situations where cooperation between cells is essential, such as in formation of communities, symbiosis, parasitism or pathogenicity.…”