THE 2 SINGLE-STRAND CLEAVAGES AT EACH END OF TN10 OCCUR IN A SPECIFICORDER DURING TRANSPOSITION

During Tn10 transposition, the element is excised from the donor site by double-strand cleavages at the two transposon ends. Double-strand c leavage is a central step in the nonreplicative transposition reaction of posons in both prokaryotes and eukaryotes. Evidence is presented t o show that the Tn10 double-strand cut is made by an ordered, sequenti al cleavage of the two strands. The transferred strand is cut first, a nd then the nontransferred strand is cleaved. The single-strand nicked intermediate is seen to accumulate when Mn2+ is substituted for Mg2in the reaction or when certain mutant transposases are used. The fact that the transferred strand is cleaved before the nontransferred stra nd implies that the order of strand cleavages is not the determining f actor that precludes a replicative mechanism of transposition.