CH391L/S12/ToxinAntitoxins
Toxin-Antitoxin (TA) Systems
TA systems are gene addiction constructs that occur naturally in prokaryotes, eukaryotes, and archaea. They consist of two coexpressed proteins: a stable toxin and a labile antitoxin that inhibits the detrimental effects of the toxin on the cell. If expression of the TA gene cassette is inhibited or the genes are purged from cell (from losing a plasmid carrying the TA genes, for example), the labile antitoxin in the cell degrades faster than the stable toxin. The newly uninhibited toxin then exerts its affect on the cell, resulting in growth inhibition or cell death. TA systems work by a wide variety of mechanisms, delineated into three major classes based on the interaction of toxin and antitoxin. TA systems can be located in selfish plasmids or chromosomally; TA systems in plasmids tend to result in cell death whereas chromosomally located TAs tend to be bacteriostatic. The exact mechanisms of action vary greatly between TA systems and a number of reviews have attempted to address most of their variation [1].
Type I Systems
In Type I systems, regulation of the toxin occurs on the RNA level. Antitoxins in this class are antisense RNA that repress the translation of the toxin by binding to its mRNA transcript.
The canonical example of a Type I TA system is the hok-sok system, originally identified in the E. coli plasmid R1 in 1986 [2]. In this system, both the toxin (hok) and antitoxin (sok) are expressed on the plasmid. Left by itself, hok protein results in membrane depolarization, the cessation of respiration, and cell death in most gram-negative bacteria. After transcription, hok mRNA is functionally inactive due to extensive secondary structure. Upon processing of the 3' end of hok transcript, two alternative pathways exist. Sok RNA is complementary to a large portion of the hok transcript. If sok transcripts, it will bind to the hok transcript and the pair will be degraded in an RNase III dependent manner. If sok transcripts are unavailable, hok transcripts will be accessible to translation machinery and result in the production of the toxic hok protein [3].
Type II Systems
In Type II Systems, antitoxin proteins bind to and inhibit toxin proteins.
The canonical example of a Type II TA system is the ccd system isolated from the F plasmid. The toxin in the ccd system, ccdB, causes cell death by acting as a gyrase poison. The ccdB protein binds to and stabilizes the DNA-gyrase complex and a cleavage intermediate and causes double stranded breaks in a replicating genome [4]. The coexpressed ccdA antitoxin protein binds to the ccdB protein and inhibits its toxic mechanism. ccdA, however, is constantly being degraded by ATP-dependant Lon protease. If expression of the ccd operon stops, the ccdA population in a cell will rapidly degrade and free the stable ccdB protein to exert its toxic affects.
Type III Systems
With only one example recently characterized, type III systems work by a small RNA antitoxin directly binding to and inhibiting the protein toxin. While the exact toxic mechanism has yet to be extrapolated, the ToxIN abortive infection system's ToxN has been shown to inhibit bacterial growth and is counteracted directly by ToxI RNA [5]. Through its altruistic limiting the spread of a phage infection through a population, the ToxIN system demonstrates the functional versatility TA systems can have.
Restriction Modification Systems
A variant of classical TA systems are restriction modification (RM) systems. RM systems involve two genes, a restriction endonuclease and a DNA methyltransferase. Here, the endonuclease acts as the toxin by cleaving non-methylated restriction sites in the host's genome wherease the methyltransferase acts as the antitoxin by protecting the endonuclease's target sites by methylation. If the source of the RM genes are lost, such as through replacement of chromosomal genes from incoming DNA sources, then the subsequent daughter cells of such a cell would have diluted and constantly degrading amount of both RM proteins. If the concentration of the DNA methyltransferase becomes low enough, it will not be able to methylate all of the endonuclease's restriction sites, resulting in DNA cleavage and possibly cell death.
Metabolism Based Plasmid Addiction
While TA systems are a
Operator Repressor Titration Systems
References
- Hayes F and Van Melderen L. Toxins-antitoxins: diversity, evolution and function. Crit Rev Biochem Mol Biol. 2011 Oct;46(5):386-408. DOI:10.3109/10409238.2011.600437 |
Toxins-antitoxins: diversity, evolution and function.
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Gerdes K, Rasmussen PB, Molin S. (1986) Unique type of plasmid maintenance function: Postsegregational killing of plasmid-free cells. Proc Natl Acad Sci USA 83:3116-3120
Unique type of plasmid maintenance function: Postsegregational killing of plasmid-free cells.
- Gerdes K, Gultyaev AP, Franch T, Pedersen K, and Mikkelsen ND. Antisense RNA-regulated programmed cell death. Annu Rev Genet. 1997;31:1-31. DOI:10.1146/annurev.genet.31.1.1 |
Antisense RNA-regulated programmed cell death
- Van Melderen L. Molecular interactions of the CcdB poison with its bacterial target, the DNA gyrase. Int J Med Microbiol. 2002 Feb;291(6-7):537-44. DOI:10.1078/1438-4221-00164 |
Molecular interactions of the CcdB poison with its bacterial target, the DNA gyrase.
- Fineran PC, Blower TR, Foulds IJ, Humphreys DP, Lilley KS, and Salmond GP. The phage abortive infection system, ToxIN, functions as a protein-RNA toxin-antitoxin pair. Proc Natl Acad Sci U S A. 2009 Jan 20;106(3):894-9. DOI:10.1073/pnas.0808832106 |
