Integrons are genetic mechanisms that allow bacteria to adapt and evolve rapidly through the stockpiling and expression of new genes. These genes are embedded in a specific genetic structure called

gene cassette In biology, a gene cassette is a type of mobile genetic element that contains a gene and a recombination site. Each cassette usually contains a single gene and tends to be very small; on the order of 500–1000 base pairs. They may exist incorpora ...

(a term that is lately changing to integron cassette) that generally carries one promoterless

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 ...

(ORF) together with a recombination site (''attC''). Integron cassettes are incorporated to the ''attI'' site of the integron platform by

site-specific recombination Site-specific recombination, also known as conservative site-specific recombination, is a type of genetic recombination in which DNA strand exchange takes place between segments possessing at least a certain degree of sequence homology. Enzymes kno ...

reactions mediated by the integrase.

Discovery

Integrons were initially discovered on conjugative plasmids through their role in antibiotic resistance. Indeed, these mobile integrons, as they are now known, can carry a variety of cassettes containing genes that are almost exclusively related to antibiotic resistance. Further studies have come to the conclusion that integrons are chromosomal elements, and that their mobilisation onto plasmids has been fostered by transposons and selected by the intensive use of antibiotics. The function of the majority of cassettes found in chromosomal integrons remains unknown.

Structure

An integron is minimally composed of: * a gene encoding for a site-specific recombinase: ''intI, ''belonging to the

integrase Retroviral integrase (IN) is an enzyme produced by a retrovirus (such as HIV) that integrates—forms covalent links between—its genetic information into that of the host cell it infects. Retroviral INs are not to be confused with phage int ...

family * a proximal recombination site: ''attI'', which is recognized by the integrase and at which gene cassettes may be inserted * a promoter: ''Pc'', which directs transcription of cassette-encoded genes

Gene cassettes

Additionally, an integron will usually contain one or more gene cassettes that have been incorporated into it. The gene cassettes may encode genes for

antibiotic resistance Antimicrobial resistance (AMR) occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance. ...

, although most genes in integrons are uncharacterized. An ''attC'' sequence (also called 59-be) is a repeat that flanks cassettes and enables cassettes to be integrated at the ''attI'' site, excised and undergo

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...

Occurrence

Integrons may be found as part of

mobile genetic elements Mobile genetic elements (MGEs) sometimes called selfish genetic elements are a type of genetic material that can move around within a genome, or that can be transferred from one species or replicon to another. MGEs are found in all organisms. In ...

such as

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 ...

and

transposon A transposable element (TE, transposon, or jumping gene) is a nucleic acid sequence in DNA that can change its position within a genome, sometimes creating or reversing mutations and altering the cell's genetic identity and genome size. Transpo ...

s. Integrons can also be found in

chromosomes A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...

Terminology

The term ''super-integron'' was first applied in 1998 (but without definition) to the integron with a long cassette array on the small chromosome of ''

Vibrio cholerae ''Vibrio cholerae'' is a species of Gram-negative, facultative anaerobe and comma-shaped bacteria. The bacteria naturally live in brackish or saltwater where they attach themselves easily to the chitin-containing shells of crabs, shrimps, and oth ...

''. The term has since been used for integrons of various cassette array lengths or for integrons on bacterial chromosomes (versus, for example, plasmids). Use of "super-integron" is now discouraged since its meaning is unclear. In more modern usage, an integron located on a bacterial chromosome is termed a ''sedentary chromosomal integron'', and one associated with transposons or plasmids is called a ''mobile integron''.

References

Medical terms

External links

IntegronFinder
- A tool to detect integrons in bacterial genomes
INTEGRALL
- The Integron Database Genetics Bacteriology Enzymes {{genetics-stub