Intermolecular and intramolecular transposition and transposition immunity in Tn3 and Tn2660

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Abstract
Intermolecular transposition of Tn2660 into pCR1 was measured at 30°C in recA − and recA + hosts as between 2.6 and 5.5x10−3, a similar value to that previously found for Tn3. No cointegrate structures were found under conditions where 104 transposition events occurred. Immunity to intermolecular transposition of Tn2660, similar to that found for Tn3 was demonstrated by showing that the above transposition frequency was reduced by a factor of between 10−3 and 10−4 when a mutant Tn2660 (resulting in the synthesis of a temperaturesensitive β-lactamase) was present in the recipient plasmid. Intramolecular transposition of Tn3 was found to occur under the same conditions as previously demonstrated for Tn2660 giving rise to similar end products, in which the newly introduced Tn3 is oriented inversely to the resident Tn3 and the DNA sequence between the two transposons has been inverted. Thus, in all respects functional identity of the transposition activities of Tn3 and Tn2660 is shown, thereby identifying characteristics of intramolecular transposition that are not readily accommodated by current models of transposition.