Addiction modules are

toxin-antitoxin system A toxin-antitoxin system is a set of two or more closely linked genes that together encode both a "toxin" protein and a corresponding "antitoxin". Toxin-antitoxin systems are widely distributed in prokaryotes, and organisms often have them in mult ...

s. Each consists of a pair of

gene In biology, the word gene (from , ; "...Wilhelm Johannsen coined the word gene to describe the Mendelian units of heredity..." meaning ''generation'' or ''birth'' or ''gender'') can have several different meanings. The Mendelian gene is a ba ...

s that specify two components: a stable toxin and an unstable antitoxin that interferes with the lethal action of the toxin. Found first in

E. coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...

on low copy number

plasmids A plasmid is a small, extrachromosomal DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently. They are most commonly found as small circular, double-stranded DNA molecules in bacteria; how ...

, addiction modules are responsible for a process called the postsegregational killing effect. When

bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among ...

lose these plasmid(s) (or other extrachromosomal elements), the cured

cell Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery ...

s are selectively killed because the unstable antitoxin is degraded faster than the more stable toxin. The term "addiction" is used because the cell depends on the

de novo synthesis In chemistry, ''de novo'' synthesis () refers to the synthesis of complex molecules from simple molecules such as sugars or amino acids, as opposed to recycling after partial degradation. For example, nucleotides are not needed in the diet as the ...

of the antitoxin for cell survival. Thus, addiction modules are implicated in maintaining the stability of extrachromosomal elements.

Proteic Addiction Modules

Proteic addiction modules use proteins as toxins and antitoxins, as opposed to RNA or other methods. The known proteic addiction modules all have similar shared characteristics, including placement of the antitoxin gene relative to the toxin gene, method of toxin neutralization by the antitoxin, and autoregulation of the addiction module by the antitoxin or toxin:antitoxin complex.

Transcriptional control of antitoxin:toxin ratios

In protein-based addiction modules, the genes encoding the toxin and antitoxin lie adjacent to each other and are continuously expressed under one

operon In genetics, an operon is a functioning unit of DNA containing a cluster of genes under the control of a single promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo splic ...

. To ensure survival of the host when the addiction module is present, more antitoxin must be produced than toxin (to counter the shorter lifespan of the antitoxin molecules). Safe ratios of the toxin and antitoxin are maintained at least in part by both this overexpression and by having the antitoxin-encoding gene encoded upstream from the toxin gene, so that the antitoxin is available to immediately neutralize the toxin. This upstream placement of the antitoxin gene is found in all proteic addiction modules. In addition, the transcription of the whole addiction module is often negatively autoregulated (i.e. the presence of its products decreases its transcriptional rate) by the formation of toxin:antitoxin complexes.

Characteristics of antitoxin molecules

The antitoxin is generally less stable than the toxin due to its degradation by

proteases A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the for ...

already present in the cell. For example, in the ccdAB proteic addiction module, the ''Lon'' proteas

degrades the antitoxin, but also serves many unrelated proteolytic roles, such as degrading oxidated mitochondrial products. This may indicate that the development of these addiction molecules "co-opted" existing cell utilities. The antitoxin in proteic addiction modules functions by binding directly to the toxin and preventing its mode of action. Once the antitoxin has bound to the toxin, the toxin prevents the proteases normally responsible for degrading antitoxin to do so, maintaining the neutralization of that individual toxin molecule.

Antisense RNA addiction modules

Antisense RNA-type addiction modules use a regulatory strand of RNA which is at least partially "antisense" (having complementary base pair encoding) to bind to toxin RNA, and thus prevent toxin translation. This antisense RNA molecule plays the role of antitoxin, similar to the proteic equivalent described above, and is similarly degraded at a faster rate than the toxin mRNA it inhibits. In addition, the transcription of the antitoxin RNA is heavily upregulated by a strong Promoter (biology), promoter which ensures excess antitoxin in cells which have a functioning addiction module.

Examples

Hok/sok system The hok/sok system is a postsegregational killing mechanism employed by the R1 plasmid in ''Escherichia coli''. It was the first type I toxin-antitoxin pair to be identified through characterisation of a plasmid-stabilising locus. It is a type I sy ...

: The transcription of ''sok'' (suppression of killing) RNA allows it to bind to a region that overlaps the

open reading frame In molecular biology, open reading frames (ORFs) are defined as spans of DNA sequence between the start and stop codons. Usually, this is considered within a studied region of a prokaryotic DNA sequence, where only one of the six possible readin ...

of the ''hok'' (host killing) toxin RNA. * ''Par'' stability determinant: Two small RNAs are transcribed simultaneously from opposite ends of a gene towards a bi-directional terminator. The two products, RNA I (toxin) and RNA II (antitoxin) immediately form a stable complex where RNA II binds (and occludes) the

ribosome binding site A ribosome binding site, or ribosomal binding site (RBS), is a sequence of nucleotides upstream of the start codon of an mRNA transcript that is responsible for the recruitment of a ribosome during the initiation of translation. Mostly, RBS refers t ...

of RNA I, preventing translation of RNA I and thus production of toxin.

References

{{cite journal , last = Shokeen , first = Sonia , author2=Greenfield, Tony J, author3= Ehli, Erik A, author4= Rasmussen, Jessica, author5= Perrault, Brian E, author6= Weaver, Keith E. , title = An Intramolecular Upstream Helix Ensures the Stability of a Toxin-Encoding RNA in Enterococcus faecalis , journal = Journal of Bacteriology , volume = 191 , pages = 1528–1536 , publisher = American Society for Microbiology , date = March 2009 , url= , doi =10.1128/JB.01316-08 , issue=5 , pmid=19103923 , pmc=2648210 Bacteriology

Proteic Addiction Modules

Transcriptional control of antitoxin:toxin ratios

Characteristics of antitoxin molecules

Antisense RNA addiction modules

Examples

See also

References