Resolution of cointegrates between transposons gamma delta and Tn3 defines the recombination site

Transposition of the genetically related insertion elements gamma delta and Tn3 is thought to involve two steps. In the case of transposition from one replicon to another, the first step is fusion of the parent and target replicons with the element appearing in direct orientation at the two junctions. In a subsequent reaction, the cointegrate structure is resolved via a site-specific recombination event. I have constructed two plasmids, each carrying segments of gamma delta and Tn3, that contain the internal resolution site. The tnpR gene product encoded by either Tn3 or gamma delta mediates intramolecular recombination between these two sites. The product of this recombination is a hybrid region that contains gamma delta and Tn3 sequences fused at the point of crossover. DNA sequence analysis of such recombinants indicates that the recombination occurs within a 19-base-pair (bp) region of exact homology between gamma delta and Tn3. The site lies in the 160-bp center intercistronic region, 50 bp before the beginning of the tnpA gene. My results therefore suggest a model for the coupled regulation of the repressor (tnpR) and the transposase (tnpA) genes and site-specific recombination of transposition intermediates. The Tn3/gamma delta recombination system and bacteriophage lambda integration are compared.