NON-PALINDROMIC ATTI SITES OF INTEGRONS ARE CAPABLE OF SITE-SPECIFIC RECOMBINATION WITH ONE ANOTHER AND WITH SECONDARY TARGETS

Genes borne on cassettes are mobile owing to site-specific recombinati on systems called integrons, which have created various combinations o f antibiotic resistance genes in R-plasmids. In these processes, the p alindromic site, attC (59-base element), at cassette junctions has bee n proposed as being essential. Excised and circularized cassettes have been found to integrate with preference for an attl site at one end o f the conserved sequence in integrons. In this work, we give evidence that recombination is possible in the absence of the highly organized attC sites between the more simply organized attl sites. Furthermore, at a very low frequency representing the background in our recombinati on assay, we observed cross-overs between attl and secondary sites. To characterize recombination excluding the attC sites, we have used nat urally occurring attl variants and constructed mutants. The cross-over point was identified between a guanine and a thymine in attl using po int mutations. progressive deletions showed the extent of attl and ide ntified two important regions in the conserved sequence 5' of the cros s-over point. A region 27-36 bp 5' of attl influenced recombination wi th attC sites only, whereas a sequence 9-14 bp 5' of the cross-over po int in attl was important for recombination with both attl and attC. R ecombination between attl and secondary sites could allow fusion of th e conserved sequence encoding the integron site-specific recombinase t o new sequences.