CHARACTERIZATION OF SPECIFIC AND SECONDARY RECOMBINATION SITES RECOGNIZED BY THE INTEGRON DNA INTEGRASE

Integrons determine a site-specific recombination system which is resp onsible for the acquisition of genes, particularly antibiotic resistan ce genes. The integrase encoded by integrons recognises two distinct c lasses of recombination sites. The first is the family of imperfect in verted repeats, known as 59-base elements, which are associated with t he mobile gene cassettes. The second consists of a single site into wh ich the cassettes are inserted. This site, here designated attl, is lo cated adjacent to the inf gene in the recipient integron structure. Th e attl site has none of the recognisable features of members of the 59 -base element family except for a seven-base core site, GTTRRRY, at th e recombination crossover point. Using a conduction assay to quantitat e site activity, the sequence required for maximal attl site activity was confined to a region of > 39 and less than or equal to 70 bases. B oth integrative and excisive site-specific recombination events involv ing attl and a 59-base element site were demonstrated, but no evidence for events involving two attl sites was obtained. Integrase-mediated recombination between a 59-base element and several secondary sites in pACYC184 with the consensus GNT occurred at low frequency, and such e vents could potentially lead to insertion of gene cassettes at many no n-specific sites.