Mobile gene cassettes and integrons: capture and spread of genes by site‐specific recombination

Abstract

SummaryAn integron is a genetic unit that includes the determinants of the components of a site-specific recombination system capable of capturing and mobilizing genes that are contained in mobile elements called gene cassettes. An integron also provides a promoter for expression of the cassette genes, and integrons thus act both as natural cloning systems and as expression vectors. The essential components of an integron are an int gene encoding a site-specific recombinase belonging to the integrase family, an adjacent site, attl, that is recognized by the integrase and is the receptor site for the cassettes, and a promoter suitably oriented for expression of the cassette-encoded genes. The cassettes are mobile elements that include a gene (most commonly an antibiotic-resistance gene) and an integrase-specific recombination site that is a member of a family of sites known as 59-base elements. Cassettes can exist either free in a circularized form or integrated at the attl site, and only when integrated is a cassette formally part of an integron. A single site-specific recombination event involving the integron-associated attl site and a cassette-associated 59-base element leads to insertion of a free circular cassette into a recipient integron. Multipie cassette insertions can occur, and integrons containing several cassettes have been found in the wild. The integrase also catalyses excisive recombination events that can lead to loss of cassettes from an integron and generate free circular cassettes. Due to their ability to acquire new genes, integrons have a clear role in the evolution of the genomes of the piasmids and transposons that contain them. However, a more general role in evoiution is also likely. Events involving recombination Received 4 July, 1994; revised 17 October, 1994; accepted 19 Oclober, 1994 between a specific 59-base-element site and a nonspecific secondary site have recently been shown to occur. Such events should lead either to the insertion of cassettes at non-specific sites or to the formation of stable cointegrates t>etween different plasmid molecules, and a cassette situated outside the integron context has recently been identified.

Emergence of Salmonella epidemics: The problems related to Salmonella enterica serotyp Enteritidis and multiple antibiotic resistance in other major serotypes

“…The dfrA1 and orf gene cassettes may have been introduced by homologous recombination with a class 1 integron containing the same array of gene cassettes from another bacterium [116]. Another possibility is the exchange between aadA2 and the two gene cassettes which would imply excision, mediated by the integron-encoded integrase, of aadA2 and its replacement by the other gene cassettes [64]. The array of gene cassettes found at the first attI1 site of the S. Albany strain was interestingly the same as those recently reported in integrons from Vibrio cholerae isolated in Thailand and India [36,142].…”

MOLECULAR DETECTION OF ANTIBIOTIC-RESISTANCE DETERMINANTS INESCHERICHIA COLIISOLATED FROM THE ENDANGERED AUSTRALIAN SEA LION (NEOPHOCA CINEREA)

“…Mobile DNA elements (integrons) have been particularly important in emergence of antibiotic resistance. Integrons encode for integrase genes (int) that enable insertion of gene cassettes at a recombination site (attI) and subsequent expression at an adjacent promotor (P c ) (Hall and Collis 1995;Collis et al 1998). Integrons can be classified into classes I, II, and III, with class I being most common in clinical pathogens (Stokes and Hall 1989).…”

Novel Class 1 Integrons in Multi-drug Resistant Isolates from Eastern China

“…The 5 0 CS contains an integrase gene coding for site-specific integrase (intI) and a recombination site (attI) involving gene cassette site-specific recombination [6]. The 3 0 CS includes the truncated qacE and sul genes that encode resistance to quaternary ammonium compounds and sulfonamides, respectively [7].…”

“…The 3 0 CS includes the truncated qacE and sul genes that encode resistance to quaternary ammonium compounds and sulfonamides, respectively [7]. The variable region consists of one or more gene cassettes captured by integron through site-specific recombination mechanism [6].…”