Integron
Encyclopedia
An integron is a two component gene capture and dissemination system, initially discovered in relation to antibiotic resistance, and which is found in plasmids, chromosomes and transposons. The first component consists of a gene encoding a site specific recombinase along with a specific site for recombination, while the second component comprises fragments of DNA
called gene cassette
s which can be incorporated or shuffled.
An integron with a large number of cassettes may be called a super-integron, as in Vibrio cholerae
chromosome 2. A cassette may encode genes for antibiotic resistance
, although most genes in integrons are uncharacterized. An integron contains an integrase
(int1) related to those of a phage, followed by an attI site for integration of cassettes and recognition of the integrase, and a promoter to drive expression. An integron may appear in a plasmid
or on the chromosome. 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
.
A functional integron "platform" requires:
Superintegrons is an incorrect term to call an integron platform associated with long cassette arrays. Most of the integron platforms that contain hundreds of gene cassettes such as in Vibrio cholerae are found as part of the bacterial chromosome. Therefore the integron nomeclature that depicts them should be as chromosomal integrons and not superintegrons and those that mobilise on plasmids or transposons and contain small numbers of gene cassettes or no gene cassettes as mobile integrons.
Abstract:
Class 1 integrons play a role in the emergence of multi-resistant bacteria by facilitating the recruitment of gene cassettes encoding antibiotic resistance genes. 512 E. coli strains sourced from humans (n = 202), animals (n = 304) and the environment (n = 6) were screened for the presence of the intI1 gene. In 31/79 integron positive E. coli strains, the gene cassette regions could not be PCR amplified using standard primers. DNA sequence analysis of 6 serologically diverse strains revealed atypical integrons harboured the dfrA5 cassette gene and only 24 bp of the integron 3′-conserved segment (CS) remained, due to the insertion of IS26. PCR targeting intI1 and IS26 followed by restriction fragment length polymorphism (RFLP) analysis identified the integron-dfrA5-IS26 element in 27 E. coli strains of bovine origin and 4 strains of human origin. Southern hybridization and transformation studies revealed the integron-dfrA5-IS26 gene arrangement was either chromosomally located or plasmid borne. Plasmid location in 4/9 E. coli strains and PCR linkage of Tn21 transposition genes with the intI1 gene in 20/31 strains, suggests this element is readily disseminated by horizontal transfer.
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
called gene cassette
Gene cassette
A gene cassette is broadly a modular DNA sequence encoding one or more genes for a single biochemical function.In genetic engineering, a gene cassette refers to a manipulable fragment of DNA carrying, and capable of expressing, one or more genes of interest between one or more sets of restriction...
s which can be incorporated or shuffled.
An integron with a large number of cassettes may be called a super-integron, as in Vibrio cholerae
Vibrio cholerae
Vibrio cholerae is a Gram-negative, comma-shaped bacterium. Some strains of V. cholerae cause the disease cholera. V. cholerae is facultatively anaerobic and has a flagella at one cell pole. V...
chromosome 2. A cassette may encode genes for antibiotic resistance
Antibiotic resistance
Antibiotic resistance is a type of drug resistance where a microorganism is able to survive exposure to an antibiotic. While a spontaneous or induced genetic mutation in bacteria may confer resistance to antimicrobial drugs, genes that confer resistance can be transferred between bacteria in a...
, although most genes in integrons are uncharacterized. An integron contains an integrase
Integrase
Retroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
(int1) related to those of a phage, followed by an attI site for integration of cassettes and recognition of the integrase, and a promoter to drive expression. An integron may appear in a plasmid
Plasmid
In microbiology and genetics, a plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double-stranded and, in many cases, circular...
or on the chromosome. 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
Horizontal gene transfer , also lateral gene transfer , is any process in which an organism incorporates genetic material from another organism without being the offspring of that organism...
.
A functional integron "platform" requires:
- an integraseIntegraseRetroviral integrase is an enzyme produced by a retrovirus that enables its genetic material to be integrated into the DNA of the infected cell...
: intI, a tyrosine recombinase responsible for incorporation into the genome - a proximal recombinationGenetic recombinationGenetic recombination is a process by which a molecule of nucleic acid is broken and then joined to a different one. Recombination can occur between similar molecules of DNA, as in homologous recombination, or dissimilar molecules, as in non-homologous end joining. Recombination is a common method...
site: attI, which acts as the locus for reincorporation to the genome. It combines with a attC site at the insertion site.
Further reading
- INTEGRALL The Integron Database
- Journal of Bacteriology, June 2002, p. 3017-3026, Vol. 184, No. 11 article Characterization of the Class 3 Integron and the Site-Specific Recombination System It Determines
- NCBI Antimicrob Agents Chemother. 1998 Dec;42(12):3053-8. article Class 1 integron-borne multiple-antibiotic resistance carried by IncFI and IncL/M plasmids in Salmonella enterica serotype typhimurium
- Nature Reviews Microbiology 4, 608-620. Mazel, D. Integrons: agents of bacterial evolution
Superintegrons is an incorrect term to call an integron platform associated with long cassette arrays. Most of the integron platforms that contain hundreds of gene cassettes such as in Vibrio cholerae are found as part of the bacterial chromosome. Therefore the integron nomeclature that depicts them should be as chromosomal integrons and not superintegrons and those that mobilise on plasmids or transposons and contain small numbers of gene cassettes or no gene cassettes as mobile integrons.
- http://www.plosone.org/article/info:doi/10.1371/journal.pone.0012754 Dawes FE, Kuzevski A, Bettelheim KA, Hornitzky MA, Djordjevic SP, Walker MJ, 2010 Distribution of Class 1 Integrons with IS26-Mediated Deletions in Their 3′-Conserved Segments in Escherichia coli of Human and Animal Origin. PLoS ONE 5(9): e12754. doi:10.1371/journal.pone.0012754
Abstract:
Class 1 integrons play a role in the emergence of multi-resistant bacteria by facilitating the recruitment of gene cassettes encoding antibiotic resistance genes. 512 E. coli strains sourced from humans (n = 202), animals (n = 304) and the environment (n = 6) were screened for the presence of the intI1 gene. In 31/79 integron positive E. coli strains, the gene cassette regions could not be PCR amplified using standard primers. DNA sequence analysis of 6 serologically diverse strains revealed atypical integrons harboured the dfrA5 cassette gene and only 24 bp of the integron 3′-conserved segment (CS) remained, due to the insertion of IS26. PCR targeting intI1 and IS26 followed by restriction fragment length polymorphism (RFLP) analysis identified the integron-dfrA5-IS26 element in 27 E. coli strains of bovine origin and 4 strains of human origin. Southern hybridization and transformation studies revealed the integron-dfrA5-IS26 gene arrangement was either chromosomally located or plasmid borne. Plasmid location in 4/9 E. coli strains and PCR linkage of Tn21 transposition genes with the intI1 gene in 20/31 strains, suggests this element is readily disseminated by horizontal transfer.