TisB-IstR toxin-antitoxin system
|Conserved secondary structure of IstR sRNA.|
|TisB Type I toxin-antitoxin system|
IstR-1 and IstR-2
IstR sRNA (inhibitor of SOS-induced toxicity by RNA) is a family of non-coding RNA first identified in Escherichia coli. There are two small RNAs encoded by the IstR locus: IstR-1 and IstR-2, of which IstR-1 works as antitoxins against the toxic protein TisB (toxicity-induced by SOS B) which is encoded by the neighbouring tisAB gene. IstR-1 is a 75 nucleotide transcript expressed constitutively throughout growth, whereas IstR-2 is a 140 nucleotide transcript induced by Mitomycin C (MMC). Both genes are thought to be regulated by LexA.
Deletion analysis confirmed the function of IstR, E. coli strain K-12 could not grow in the absence of IstR when tisAB was present. Inserting IstR genes on a plasmid allowed the bacteria to grow normally. Further studies showed that expression of IstR-1 alone is enough to remedy the toxic effects of TisB. IstR-2 is not involved in the regulation of tisAB.
The tisAB locus codes for two genes: tisA and tisB. The tisA reading frame was shown through a translation assay to not be translated. Its sequence is unconserved across species. TisB is a 29 amino acid peptide widely conserved in enterobacteria. TisB is responsible for conferring toxicity through suspected membrane disruption. Upon translation of the tisB gene, a +1 inactive primary transcript mRNA is produced, which must be endonucleolytically processed 42 nucleotides from the 5' end to yield a +42 translationally competent mRNA. In the +42 form, the mRNA has a ribosome loading/standby site in an unstructured region >80 nt upstream of the tisB ribosome binding site, thus allowing translation of the TisB protein. This standby site is structurally unavailable in the inactive forms of the tisB mRNA (the +1 form and the +106 form produced by RNase III cleavage).
Mechanism of TisB inhibition by IstR-1
IstR-1 is thought to both inhibit translation of the TisB toxin, and promote RNase III cleavage of the RNA duplex formed when IstR-1 base pairs to tisB mRNA. Binding of the complimentary sequence of istR-1 sRNA to tisB mRNA in the ribosome standby site is thought to prevent loading of ribosomes and therefore prevent translation of the TisB protein. A RACE analysis confirmed that IstR-1 binds TisB mRNA and the duplex is then degraded by RNase III. Degradation results in a +106 form, an inactive 249 nt transcript which cannot be translated.
Proposed function of the IstR-TisB toxin-antitoxin system
The proposed function of this toxin-antitoxin system is to cause growth arrest, rather than cell death, in response to DNA damage, allowing time for repair processes to occur. TisB translation is under LexA control, so it is induced by DNA damage as part of the SOS response. Under normal conditions, very little tisB mRNA is synthesised and translation is inhibited, but when DNA damage occurs tisAB is strongly induced causing overexpression, which overrides inhibition by depleting the IstR-1 pool. Experimental data has shown effects of TisB to be decreases in transcription, translation and replication, RNA degradation and ribosome disassembly. TisB does not affect transcription and translation directly in vitro, so these effects are thought to be downstream consequences of membrane damage.
TisB insertion into the membrane is thought to result in a loss of membrane potential. This could account for a decrease in ATP concentration in cells following triggering of the SOS response, causing slowing of cellular processes and inhibited cell growth.
- Vogel J, Argaman L, Wagner EG, Altuvia S (December 2004). "The small RNA IstR inhibits synthesis of TisB, SOS-induced toxic peptide". Curr. Biol. 14 (24): 2271–6. doi:10.1016/j.cub.2004.12.003. PMID 15620655. Retrieved 2010-08-11.
- Darfeuille F, Unoson C, Vogel J, Wagner EG (May 2007). "An antisense RNA inhibits translation by competing with standby ribosomes". Mol. Cell 26 (3): 381–92. doi:10.1016/j.molcel.2007.04.003. PMID 17499044. Retrieved 2010-08-11.
- Gerdes, K.; Wagner, E. (2007). "RNA antitoxins". Current opinion in microbiology 10 (2): 117–124. doi:10.1016/j.mib.2007.03.003. PMID 17376733.
- Unoson, C.; Wagner, E. G. H. (2008). "A small SOS-induced toxin is targeted against the inner membrane in Escherichia coli". Molecular Microbiology 70 (1): 258–270. doi:10.1111/j.1365-2958.2008.06416.x. PMID 18761622.
- Weel-Sneve, R.; Bjoras, M.; Kristiansen, K. I. (2008). "Overexpression of the LexA-regulated tisAB RNA in E. Coli inhibits SOS functions; implications for regulation of the SOS response". Nucleic Acids Research 36 (19): 6249–6259. doi:10.1093/nar/gkn633. PMC 2577331. PMID 18832374.
- Sharma CM, Vogel J (October 2009). "Experimental approaches for the discovery and characterization of regulatory small RNA". Curr. Opin. Microbiol. 12 (5): 536–46. doi:10.1016/j.mib.2009.07.006. PMID 19758836. Retrieved 2010-08-11.
- Wassarman KM, Repoila F, Rosenow C, Storz G, Gottesman S (July 2001). "Identification of novel small RNAs using comparative genomics and microarrays". Genes Dev. 15 (13): 1637–51. doi:10.1101/gad.901001. PMC 312727. PMID 11445539. Retrieved 2010-08-11.
- Santiviago CA, Reynolds MM, Porwollik S, et al. (July 2009). "Analysis of pools of targeted Salmonella deletion mutants identifies novel genes affecting fitness during competitive infection in mice". PLoS Pathog. 5 (7): e1000477. doi:10.1371/journal.ppat.1000477. PMC 2698986. PMID 19578432. Retrieved 2010-08-11.
- Fozo EM, Makarova KS, Shabalina SA, Yutin N, Koonin EV, Storz G (June 2010). "Abundance of type I toxin-antitoxin systems in bacteria: searches for new candidates and discovery of novel families". Nucleic Acids Res. 38 (11): 3743–59. doi:10.1093/nar/gkq054. PMC 2887945. PMID 20156992. Retrieved 2010-08-11.
- Rudd KE (1999). "Novel intergenic repeats of Escherichia coli K-12". Res. Microbiol. 150 (9-10): 653–64. doi:10.1016/S0923-2508(99)00126-6. PMID 10673004. Retrieved 2010-08-11.
- Wagner, E. G. H.; Unoson, C. (2012). "The toxin-antitoxin system tisB-istR1: Expression, regulation, and biological role in persister phenotypes". RNA Biology 9 (12): 1513–1519. doi:10.4161/rna.22578. PMID 23093802.