Insertions of resistance genes into Tn21-like transposons.

The evolution of Tn21 and related multiresistance transposons of natural and artificial origin is described. Variations resulting from insertions and deletions in these transposons indicate the presence of specific recombination sites. There is evidence that these 'hot spots' are flanking several resistance genes. We examined these structures on the basis of the aadA gene, mediating resistance to spectinomycin and streptomycin. Comparison of the DNA sequences of Tn21, Tn7 and pSa indicate the wide spread of these recombination sites.