The transposable element Tn3 promotes general recombination at the neighboring regions.

Transposon Tn3 was inserted into a tRNA operon of the amber suppressor Su+2 on a transducing phage (lambda hcI857nin5pSu+2) by selecting phages with ampicillin resistance and Su- phenotypes. In a strain thus obtained, Tn3 was inserted between the promoter and the first tRNA gene of the operon, which was determined by DNA sequencing. The Su+2 tRNA operon on the transducing phage consisted of two tRNA genes for tRNA(Met) and Su+2 tRNA(2Gln), which was a deletion derivative of the supB-E tRNA operon of E. coli containing seven tRNA genes in the order of promoter-Met-Leu-Gln1-Gln1-Met-Gln2-Gln2. Proliferating the lambda hcI857nin5pSu+2::Tn3 in E. coli cells, a number of phages which had lost Tn3 were isolated, and their tRNA gene compositions as well as the DNA structures of the tRNA operon were analyzed. In many cases the tRNA genes which had been deleted from the original transducing phage were regained from the chromosomal supB-E operon. Thus the loss of Tn3 from the phages was not due to excision of the transposon but due to the replacement of a portion of the tRNA operon, including Tn3, with the host homologous region that did not contain Tn3. This type of replacement takes place rather efficiently as a consequence of Tn3 insertion, owing to the general recombination occurring between homologous tRNA genes of phage and host chromosomes in the presence of either host recA or phage red. No such enhanced recombination in a similar cross between phage and host chromosomes was observed with the Tn3 present in the trans position on an independent plasmid. We conclude that inserting Tn3 in cis promotes general recombination in the neighboring regions. Possible mechanisms for this new type of genetic effect of Tn3 are discussed. During the course of this study, a natural defective mutation (T11) was also detected in one of the duplicated tRNA(2Gln) genes in an E. coli K12 strain we used.