Transposon Tn
3 was inserted into a tRNA operon of the amber suppressor Su
+2 on a transducing phage (λ
hcI857
nin5pSu
+2) by selecting phages with ampicillin resistance and Su
- phenotypes. In a strain thus obtained, Tn
3 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 transducingphage consisted of two tRNA genes for tRNA
Met and Su
+2 tRNA
Gln2, 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-Gln
1-Gln
1-Met-Gln
2-Gln
2. Proliferating the λ
hcI857
nin5pSu
+2::Tn
3 in
E. coli cells, a number of phages which had lost Tn
3 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 Tn
3 from the phages was not due to excision of the transposon but due to the replacement of a portion of the tRNA operon, including Tn
3, with the host homologous region that did not contain Tn
3. This type of replacement takes place rather efficiently as a consequence of Tn
3 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 Tn
3 present in the
traps position on an indepedent plasmid. We conclude that inserting Tn
3 in
cis promotes general recombination in the neighboring regions. Possible mechanisms for this new type of genetic effect of Tn
3 are discussed. During the course of this study, a natural defective mutation (T11) was also detected in one of the duplicated tRNA
Gln2 genes in an
E. coli K12 strain we used.
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