Deletions of transposons Tn1 and Tn3 that extend into a region of the transposon that specifies a 19,000 MW protein, are unable to resolve presumptive transposition intermediates in recA strains of Escherichia coli. When transposition of such mutant transposons occurs from replicon A to replicon B, cointegrate molecules containing A and B separated by directly repeated copies of the transposons are efficiently produced. Such cointegrates are stable in a recA strain, but are resolved within a recA+ host into replicons A and B each containing a copy of the transposon. One mutant gives cointegrates that can be complemented to resolve when a wild type Tn3 is present in the same recA cell, whereas another gives cointegrates that cannot be resolved by complementation in trans. The 1st such mutant still carries the sequences necessary for the recombination event whereas the latter has lost them. The presence of a Tn1/3 specified site-specific recombination system was confirmed by showing that naturally-occurring multimers of a Tn3 derivative of plasmid pMB8 can be efficiently resolved to monomers in a recA- strain, whereas dimers of pMB9 (a Tcr derivative of pMB8) and 2 deleted Tn3 derivatives of pMB8 that are defective in the production of the 19,000 MW protein, were both stably maintained as dimers in a recA- strain. Analysis of the ability of multimeric forms of other pMB8:: Tn3 deletion derivatives to be stably propagated in a recA- strain, allowed the localization of the Tn3 sequences necessary for the recombination event.
