Summary It has been well established that Tn
3 and its relatives transpose from one replicon to another by two successive reactions: formation of the cointegrate molecule and resolution from it. Whether or not the 9300 base pair tetracycline resistance transposon Tn
10 transposes in the same manner as Tn
3 was investigated by two methods.In the first method, 55, a lambda phage carrying Tn
10 was lysogenized in an
Escherichia coli strain carrying a Tn
10 insertion; the phage has a deletion in
attP, hence it was lysogenized in a Tn
10 sequence in the
E. coli chromosome by reciprocal recombination. The chromosomal structure in these lysogens is equivalent to the Tn
10-mediated cointegrate molecule of lambda and the
E. coli chromosomal DNA. The stability of the cointegrate molecule was examined by measuring the rate of excision of lambda from the host chromosome, and was found to be stable, especially in a Rec
- strain. Because of this stability, the cointegrate molecule should be accumulated if Tn
10 transposes via the cointegrate molecule. Then, we examined the configuration of products made by transposition of Tn
10 from 55 to the
E. coli chromosome. The cointegrate molecule was found in products of Tn
10 transposition in a Rec
+ strain at a frequency of 5% per Tn
10 transposition, but this molecule could not be found in a Rec
- strain. Since transposition of Tn
10 was
recA-independent, absence of the cointegrate molecule formed in a RecA
- strain strongly suggested that the cointegrate molecule is not an obligatory intermediate of transposition of Tn
10.In the second method, mobilization of pACYC177 by R388 and by R388:: Tn
10 was examined. The pACYC177 plasmid was mobilized by R388::Tn
10 at a frequency of 10
-4 per donor but not by R388. It occurred, in most cases, by inverse transposition of R388::Tn
10 to pACYC177 forming plasmids such as pACYC177::IS
10-R388-IS
10. Mobilization of pACYC177 by a Tn
10-mediated cointegrate in the form of pACYC177::Tn
10-R388-
Tn10 was not observed in crosses using a Rec
- donor. These observations also suggested that transposition of Tn
10 in Rec
- cells does not occur via the cointegrate molecule.
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