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6  structures 1107  species 0  interactions 2303  sequences 9  architectures

Family: HicB_lk_antitox (PF15919)

Summary: HicB_like antitoxin of bacterial toxin-antitoxin system

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This is the Wikipedia entry entitled "Toxin-antitoxin system". More...

Toxin-antitoxin system Edit Wikipedia article

(A) The vertical gene transfer of a toxin-antitoxin system. (B) Horizontal gene transfer of a toxin-antitoxin system. PSK stands for post-segregational killing and TA represents a locus encoding a toxin and an antitoxin.[1]

A toxin-antitoxin system is a set of two or more closely linked genes that together encode both a protein 'poison' and a corresponding 'antidote'. When these systems are contained on plasmids – transferable genetic elements – they ensure that only the daughter cells that inherit the plasmid survive after cell division. If the plasmid is absent in a daughter cell, the unstable antitoxin is degraded and the stable toxic protein kills the new cell; this is known as 'post-segregational killing' (PSK).[2][3] Toxin-antitoxin systems are widely distributed in prokaryotes, and organisms often have them in multiple copies.[4][5]

Toxin-antitoxin systems are typically classified according to how the antitoxin neutralises the toxin. In a Type I toxin-antitoxin system, the translation of messenger RNA (mRNA) that encodes the toxin is inhibited by the binding of a small non-coding RNA antitoxin to the mRNA. The protein toxin in a type II system is inhibited post-translationally by the binding of another protein antitoxin. A single example of Type III toxin-antitoxin system has been described whereby a protein toxin is bound directly by an RNA molecule.[6] Toxin-antitoxin genes are often transferred through horizontal gene transfer[7][8] and are associated with pathogenic bacteria, having been found on plasmids conferring antibiotic resistance and virulence.[1]

Chromosomal toxin-antitoxin systems also exist, some of which perform cell functions such as responding to stresses, causing cell cycle arrest and bringing about programmed cell death.[1][9] In evolutionary terms, toxin-antitoxin systems can be considered selfish DNA in that the purpose of the systems are to replicate, regardless of whether they benefit the host organism or not. Some have proposed adaptive theories to explain the evolution of toxin-antitoxin systems; for example, chromosomal toxin-antitoxin systems could have evolved to prevent the inheritance of large deletions of the host genome.[10] Toxin-antitoxin systems have several biotechnological applications, such as a method of maintaining plasmids in cell lines, targets for antibiotics, and as positive selection vectors.[11]

Evolutionary advantage

Plasmid stabilising toxin-antitoxin systems have been used as examples of selfish DNA as part of the gene centered view of evolution. It has been theorised that toxin-antitoxin loci serve only to maintain their own DNA, at the expense of the host organism.[1] Other theories propose the systems have evolved to increase the fitness of plasmids in competition with other plasmids.[12] Thus, the toxin-antitoxin system confers an advantage to the host DNA by eliminating competing plasmids in cell progeny. This theory was corroborated through computer modelling.[13] This does not, however, explain the presence of toxin-antitoxin systems on chromosomes.

A chromosome map of Sinorhizobium meliloti, with its 25 chromosomal toxin-antitoxin systems. Orange-labelled loci are confirmed TA systems[14] and green labels show putative systems.[15]

Chromosomal toxin-antitoxin systems have a number of adaptive theories explaining their success at natural selection. The simplest explanation of their existence on chromosomes is that they prevent harmful large deletions of the cell's genome, though arguably deletions of large coding regions are fatal to a daughter cell regardless.[10] MazEF, a toxin-antitoxin locus found in E. coli and other bacteria, induces programmed cell death in response to starvation, specifically a lack of amino acids.[16] This releases the cell's contents for absorption by neighbouring cells, potentially preventing the death of close relatives, and thereby increasing the inclusive fitness of the cell that perished. This is an example of altruism and how bacterial colonies resemble multicellular organisms.[13] However, the "mazEF mediated PCD" is largely refuted by several studies.[17][18][19]

Another theory states that chromosomal toxin-antitoxin systems are designed to be bacteriostatic rather than bactericidal.[20] RelE, for example, is a global inhibitor of translation during nutrient stress, and its expression reduces the chance of starvation by lowering the cell's nutrient requirements.[21] A homologue of mazF toxin called mazF-mx is essential for fruiting body formation in Myxococcus xanthus.[22] When nutrients become limiting in this swarming bacteria, a group of 50,000 cells converge into a fruiting body structure.[23] The maxF-mx toxin is a component of this nutrient-stress pathway; it enables a percentage of cells within the fruiting body to form myxospores. It has been suggested that M. xanthus has hijacked the toxin-antitoxin system, replacing the antitoxin with its own molecular control to regulate its development.[22]

It has also been proposed that chromosomal copies of plasmid toxin-antitoxin systems may serve as anti-addiction modules – a method of omitting a plasmid from progeny without suffering the effects of the toxin.[8] An example of this is an antitoxin on the Erwinia chrysanthemi genome that counteracts the toxic activity of an F plasmid toxin counterpart.[24]

Nine possible functions of toxin-antitoxin systems have been proposed. These are:[25][26]

  1. Junk – they have been acquired from plasmids and retained due to their addictive nature.
  2. Stabilisation of genomic parasites – chromosomal remnants from transposons and bacteriophages.
  3. Selfish alleles – non-addictive alleles are unable to replace addictive alleles during recombination but the opposite is able to occur.
  4. Gene regulation – some toxins act as a means of general repression of gene expression[27] while others are more specific.[28]
  5. Growth control – bacteriostatic toxins, as mentioned above, restrict growth rather than killing the host cell.[20]
  6. Persisters – some bacterial populations contain a sub-population of 'persisters' that are slow-growing, hardy individuals, which potentially insure the population against catastrophic loss.[29] At least with regard to endoribonuclease encoding Type II TA systems, their role in persistence is highly debated.[30]. What has been demonstrated by experiments and modelling [31] is that an imbalance between the level of toxin and its antitoxin, either by mutations [32][33] or by overexpression[34] results in high persistence.
  7. Programmed cell arrest and the preservation of the commons – the altruistic explanation as demonstrated by MazEF, detailed above.
  8. Programmed cell death – similar to the above function, although individuals must have variable stress survival level to prevent entire population destruction.
  9. Antiphage mechanism – when bacteriophage interrupt the host cell's transcription and translation, a toxin-antitoxin system may be activated that limits the phage's replication.[35][36]

An experiment where five TA systems were deleted from a strain of E. coli found no evidence that the TA systems conferred an advantage to the host. This result casts doubt on the growth control and programmed cell death hypotheses.[37] As of the existing knowledge in 2017, the chromosomal Type II TA systems are horizontally propagating selfish DNA which may have played a role in antiaddiction to TA encoding plasmids.[8][26]

System types

Type I

The hok/sok type I toxin-antitoxin system

Type I toxin-antitoxin systems rely on the base-pairing of complementary antitoxin RNA with the toxin's mRNA. Translation of the mRNA is then inhibited either by degradation via RNase III or by occluding the Shine-Dalgarno sequence or ribosome binding site. Often the toxin and antitoxin are encoded on opposite strands of DNA. The 5' or 3' overlapping region between the two genes is the area involved in complementary base-pairing, usually with between 19–23 contiguous base pairs.[38]

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.[9]

Type I systems sometimes include a third component. 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. This open reading frame almost entirely overlaps that of the toxin, and the translation of the toxin is dependent on the translation of this third component.[3] Thus the binding of antitoxin to toxin is sometimes a simplification, and the antitoxin in fact binds a third RNA, which then affects toxin translation.[38]

Example systems

Toxin Antitoxin Notes Ref.
Hok Sok The original and best-understood type I toxin-antitoxin system (pictured), which stabilises plasmids in a number of gram-negative bacteria [38]
fst RNAII The first type I system to be identified in gram-positive bacteria [39]
TisB IstR Responds to DNA damage [40]
LdrD RdlD A chromosomal system in Enterobacteriaceae [41]
FlmA FlmB A hok/sok homologue, which also stabilises the F plasmid [42]
Ibs Sib Discovered in E. coli intergenic regions, the antitoxin was originally named QUAD RNA [43]
TxpA/BrnT RatA Ensures the inheritance of the skin element during sporulation in Bacillus subtilis [44]
SymE SymR A chromosomal system induced as an SOS response [5]
XCV2162 ptaRNA1 A system identified in Xanthomonas campestris with erratic phylogenetic distribution. [45]

See also

Type II

The genetic context of a typical type II toxin-antitoxin locus, produced during a bioinformatics analysis[15]

Type II toxin-antitoxin systems are generally better-understood than type I.[38] In this system a labile protein antitoxin tightly binds and inhibits the activity of a stable toxin.[9] The largest family of type II toxin-antitoxin systems is vapBC,[46] which has been found through bioinformatics searches to represent between 37 and 42% of all predicted type II loci.[14][15]

Type II systems are organised in operons with the antitoxin protein typically being located upstream of the toxin. The antitoxin inhibits the toxin by downregulating its expression. The proteins are typically around 100 amino acids in length,[38] and exhibit toxicity in a number of ways: CcdB protein, for example, affects DNA gyrase by poisoning DNA topoisomerase II[47] whereas MazF protein is a toxic endoribonuclease that cleaves cellular mRNAs at specific sequence motifs.[48] The most common toxic activity is the protein acting as an endonuclease, also known as an interferase.[49][50]

Unlike the aforementioned toxin-antitoxin systems, DarTG is a type II system where both the toxin and the antitoxin have enzymatic activity. The DarG antitoxin does not inhibit the DarT toxin, which modifies DNA by ADP-ribosylating specific sequence motifs, but instead removes the toxic modification caused by the toxin.[51]

A third protein can sometimes be involved in type II toxin-antitoxin systems.[52] In the case of the aforementioned MazEF addiction module, in addition to the toxin and antitoxin there is a regulatory protein involved called MazG. MazG protein interacts with E. coli's Era GTPase and is described as a 'nucleoside triphosphate pyrophosphohydrolase,'[53] which hydrolyses nucleoside triphosphates to monophosphates. Later research showed that MazG is transcribed in the same polycistronic mRNA as MazE and MazF, and that MazG bound the MazF toxin to further inhibit its activity.[54]

Example systems

Toxin Antitoxin Notes Ref.
CcdB CcdA Found on the F plasmid of Escherichia coli [47]
ParE ParD Found in multiple copies in Caulobacter crescentus [55]
MazF MazE Found in E. coli and in chromosomes of other bacteria [35]
yafO yafN A system induced by the SOS response to DNA damage in E. coli [52]
HicA HicB Found in archaea and bacteria [56]
Kid Kis Stabilises the R1 plasmid and is related to the CcdB/A system [20]
Zeta Epsilon Found mostly in Gram-positive bacteria [57]
DarT DarG Found in archaea and bacteria [51]

Type III

Symbol ToxN, type III toxin-antitoxin system
Pfam PF13958

Type III toxin-antitoxin systems rely on direct interaction between a toxic protein and an RNA antitoxin. The toxic effects of the protein are neutralised by the RNA gene.[6] One example is the ToxIN system from the bacterial plant pathogen Erwinia carotovora. The toxic ToxN protein is approximately 170 amino acids long and has been shown to be toxic to E. coli. The toxic activity of ToxN is inhibited by ToxI RNA, an RNA with 5.5 direct repeats of a 36 nucleotide motif (AGGTGATTTGCTACCTTTAAGTGCAGCTAGAAATTC).[58][59] Crystallographic analysis of ToxIN has found that ToxN inhibition requires the formation of a trimeric ToxIN complex, whereby three ToxI monomers bind three ToxN monomers; the complex is held together by extensive protein-RNA interactions.[60]

Biotechnological applications

The biotechnological applications of toxin-antitoxin systems have begun to be realised by several biotechnology organisations.[11][20] A primary usage is in maintaining plasmids in a large bacterial cell culture. 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.[61][62] In large-scale microorganism processes such as fermentation, progeny cells lacking the plasmid insert often have a higher fitness than those who inherit the plasmid and can outcompete the desirable microorganisms. A toxin-antitoxin system maintains the plasmid thereby maintaining the efficiency of the industrial process.[11]

Additionally, toxin-antitoxin systems may be a future target for antibiotics. Inducing suicide modules against pathogens could help combat the growing problem of multi-drug resistance.[63]

Ensuring a plasmid accepts an insert is a common problem of DNA cloning. Toxin-antitoxin systems can be used to positively select for only those cells that have taken up a plasmid containing the inserted gene of interest, screening out those that lack the inserted gene. An example of this application comes from CcdB-encoded toxin, which has been incorporated into plasmid vectors.[64] The gene of interest is then targeted to recombine into the CcdB locus, inactivating the transcription of the toxic protein. Thus, cells containing the plasmid but not the insert perish due to the toxic effects of CcdB protein, and only those that incorporate the insert survive.[11]

Another example application involves both the CcdB toxin and CcdA antitoxin. CcdB is found in recombinant bacterial genomes and an inactivated version of CcdA is inserted into a linearised plasmid vector. A short extra sequence is added to the gene of interest that activates the antitoxin when the insertion occurs. This method ensures orientation-specific gene insertion.[64]

Genetically modified organisms must be contained in a pre-defined area during research.[63] 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.[20][65]

See also


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  48. ^ Zhang Y, Zhang J, Hoeflich KP, Ikura M, Qing G, Inouye M (October 2003). "MazF cleaves cellular mRNAs specifically at ACA to block protein synthesis in Escherichia coli". Mol. Cell. 12 (4): 913–23. doi:10.1016/S1097-2765(03)00402-7. PMID 14580342. Retrieved 2010-08-11. 
  49. ^ Christensen-Dalsgaard M, Overgaard M, Winther KS, Gerdes K (2008). "RNA decay by messenger RNA interferases". Meth. Enzymol. Methods in Enzymology. 447: 521–35. doi:10.1016/S0076-6879(08)02225-8. ISBN 978-0-12-374377-0. PMID 19161859. Retrieved 2010-08-16. 
  50. ^ Yamaguchi Y, Inouye M (2009). "mRNA interferases, sequence-specific endoribonucleases from the toxin-antitoxin systems". Prog Mol Biol Transl Sci. Progress in Molecular Biology and Translational Science. 85: 467–500. doi:10.1016/S0079-6603(08)00812-X. ISBN 978-0-12-374761-7. PMID 19215780. Retrieved 2010-08-16. 
  51. ^ a b Jankevicius, Gytis; Ariza, Antonio; Ahel, Marijan; Ahel, Ivan. "The Toxin-Antitoxin System DarTG Catalyzes Reversible ADP-Ribosylation of DNA". Molecular Cell. doi:10.1016/j.molcel.2016.11.014. 
  52. ^ a b Singletary LA, Gibson JL, Tanner EJ, et al. (December 2009). "An SOS-Regulated Type 2 Toxin-Antitoxin System". J. Bacteriol. 191 (24): 7456–65. doi:10.1128/JB.00963-09. PMC 2786605Freely accessible. PMID 19837801. Retrieved 2010-08-16. 
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Further reading

External links

  • RASTA – Rapid Automated Scan for Toxins and Antitoxins in Bacteria

This page is based on a Wikipedia article. The text is available under the Creative Commons Attribution/Share-Alike License.

This tab holds the annotation information that is stored in the Pfam database. As we move to using Wikipedia as our main source of annotation, the contents of this tab will be gradually replaced by the Wikipedia tab.

HicB_like antitoxin of bacterial toxin-antitoxin system Provide feedback

This is a family of HicB-like antitoxins.

Internal database links

This tab holds annotation information from the InterPro database.

InterPro entry IPR031807

This is a family of HicB-like antitoxins of bacterial and archaeal toxin-antitoxin systems [PUBMED:19060138].

Domain organisation

Below is a listing of the unique domain organisations or architectures in which this domain is found. More...

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Pfam Clan

This family is a member of clan Met_repress (CL0057), which has the following description:

This superfamily contains many antitoxin families, all of which carry a ribbon-helix-helix DNA-binding motif with the beta-ribbon located in and recognising the major groove of operator DNA [1].

The clan contains the following 35 members:

Arc BrnA_antitoxin CcdA CopG_antitoxin DndE DUF1778 DUF1902 DUF2540 DUF2610 HicB HicB-like_2 HicB_lk_antitox MatP_C MetJ Omega_Repress ParD ParD_antitoxin ParD_like ParG Plasmid_stab_B PSK_trans_fac RelB RepB-RCR_reg Repressor_Mnt RHH_1 RHH_3 RHH_4 RHH_5 RHH_6 RHH_7 SeqA_N TraY VAPB_antitox VapB_antitoxin VirC2


We store a range of different sequence alignments for families. As well as the seed alignment from which the family is built, we provide the full alignment, generated by searching the sequence database (reference proteomes) using the family HMM. We also generate alignments using four representative proteomes (RP) sets, the UniProtKB sequence database, the NCBI sequence database, and our metagenomics sequence database. More...

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We make a range of alignments for each Pfam-A family. You can see a description of each above. You can view these alignments in various ways but please note that some types of alignment are never generated while others may not be available for all families, most commonly because the alignments are too large to handle.

Representative proteomes UniProt
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1Cannot generate PP/Heatmap alignments for seeds; no PP data available

Key: ✓ available, x not generated, not available.

Format an alignment

Representative proteomes UniProt

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We make all of our alignments available in Stockholm format. You can download them here as raw, plain text files or as gzip-compressed files.

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You can also download a FASTA format file containing the full-length sequences for all sequences in the full alignment.

HMM logo

HMM logos is one way of visualising profile HMMs. Logos provide a quick overview of the properties of an HMM in a graphical form. You can see a more detailed description of HMM logos and find out how you can interpret them here. More...


This page displays the phylogenetic tree for this family's seed alignment. We use FastTree to calculate neighbour join trees with a local bootstrap based on 100 resamples (shown next to the tree nodes). FastTree calculates approximately-maximum-likelihood phylogenetic trees from our seed alignment.

Note: You can also download the data file for the tree.

Curation and family details

This section shows the detailed information about the Pfam family. You can see the definitions of many of the terms in this section in the glossary and a fuller explanation of the scoring system that we use in the scores section of the help pages.

Curation View help on the curation process

Seed source: SWISS-PROT
Previous IDs: none
Type: Domain
Author: Bateman A
Number in seed: 28
Number in full: 2303
Average length of the domain: 85.30 aa
Average identity of full alignment: 24 %
Average coverage of the sequence by the domain: 86.27 %

HMM information View help on HMM parameters

HMM build commands:
build method: hmmbuild -o /dev/null HMM SEED
search method: hmmsearch -Z 26740544 -E 1000 --cpu 4 HMM pfamseq
Model details:
Parameter Sequence Domain
Gathering cut-off 27.0 27.0
Trusted cut-off 27.0 27.0
Noise cut-off 26.9 26.9
Model length: 123
Family (HMM) version: 4
Download: download the raw HMM for this family

Species distribution

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Colour assignments

Archea Archea Eukaryota Eukaryota
Bacteria Bacteria Other sequences Other sequences
Viruses Viruses Unclassified Unclassified
Viroids Viroids Unclassified sequence Unclassified sequence


Align selected sequences to HMM

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This visualisation provides a simple graphical representation of the distribution of this family across species. You can find the original interactive tree in the adjacent tab. More...

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Tree controls


The tree shows the occurrence of this domain across different species. More...


Please note: for large trees this can take some time. While the tree is loading, you can safely switch away from this tab but if you browse away from the family page entirely, the tree will not be loaded.


For those sequences which have a structure in the Protein DataBank, we use the mapping between UniProt, PDB and Pfam coordinate systems from the PDBe group, to allow us to map Pfam domains onto UniProt sequences and three-dimensional protein structures. The table below shows the structures on which the HicB_lk_antitox domain has been found. There are 6 instances of this domain found in the PDB. Note that there may be multiple copies of the domain in a single PDB structure, since many structures contain multiple copies of the same protein sequence.

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