Genetic elements involved in Tn21 site-specific integration, a novel mechanism for the dissemination of antibiotic resistance genes.

Abstract

Transposon Tn2l codes for a site-specific integration system, which is probably a novel recombination mechanism, responsible for the acquisition of resistance genes in this widespread family of transposons. Using insertion and deletion mutagenesis we have identified the genetic loci of the various recombination hot-spots (RHS) and of the gene product (the integrase) that catalyses the reaction. The site of recombination has been localized in two of the RHSs to the DNA sequence GTTAG, which is present at the 3' terminm of a loosely conserved palindromic sequence of -59 bp. This 59 bp sequence, which flanks the inserted genes in a number of naturally occurring transposons, is the only element required in cis for the recombination reaction.

Gene cassettes: a new class of mobile element

“…IntIl is believed to be a member of the 1 integrase family on the basis of similarities between the conserved domains shared by members of this family of site-specific recombinases Stokes & Hall, 1989;Abremski & Hoess, 1992). IntIl has been shown to catalyse both integrative and excisive recombination events in in vivo experimental systems (Martinez & de la Cruz, 1990 ;Hall et al, 1991 ;Recchia e t al. , 1994).…”

“…The specific recombination sites known to be recognized by IntIl are the cassette-associated 59-base elements (Martinez & de la Cruz, 1990;Hall e t al., 1991) and the integron-determined integration site, attl, which is largely contained within the 5'-CS and is not obviously related to the 59-base elements (Recchia e t al., 1994) (Fig. 7).…”

“…In other experimental systems this preference has not been observed. Rather, events involving two 59-base element recombination sites have generally predominated (Martinez & de la Cruz, 1990;Hall e t al., 1991 ;Collis & Hall, 1992a). While this type of event is clearly important to effect the excision of any cassette that is not located in the first position (i.e.…”

Characterization of the nonenzymatic chloramphenicol resistance (cmlA) gene of the In4 integron of Tn1696: similarity of the product to transmembrane transport proteins

“…At the 3' end of the gene cassettes, putative stem-loop structures potentially involved in site-specific recombination are observed (59-bp elements) (20,30,52,55). The requirement for these elements in cis for site-specific recombination has been demonstrated by Martinez and de la Cruz (31).…”

“…Stokes and Hall (55) have defined the elements borne on these multiresistance plasmids and transposons as integrons, a novel family of potentially mobile DNA elements which are composed of two conserved segments between which discrete units, ordinarily antibiotic resistance genes, have been integrated as gene cassettes. The 5'-conserved segment encodes a site-specific recombinase (Int) showing active-site residue similarity with the phage integrases (31,41,55). The 3'-conserved segment encodes a sulfonamide-resistant dihydropteroate synthase (Sull [56]) and two open reading frames (ORFs) of unknown phenotype, ORF4 and ORF5 (see Fig.…”