Figure 1

Schematic organization of integrons. The functional platform of integrons is constituted by an intI gene encoding an integrase, its own promoter P _int , a cassette promoter P _C , and a primary recombination site attI. The system maintains an array that can consist of more than 200 cassettes. Only the few first cassettes are strongly expressed by the P _C promoter, as indicated by the fading fill color. Cassettes generally contain a promoterless open reading frame (ORF) flanked by two recombination sites termed attC. Cassettes can be excised from any position in the array through attC × attC recombination mediated by the integrase. The resulting circular intermediate can then be integrated by the integrase, preferentially at attI. Exogenous circular intermediates can also be integrated, owing to the low specificity of the integrase activity, rendering the system prone to horizontal transfer. The SOS response directly controls the expression of many integron integrases by binding of its repressor protein, LexA, to a target site in the P _int promoter.