TN10 TRANSPOSITION VIA A DNA HAIRPIN INTERMEDIATE

We present evidence that excision of the nonreplicative transposon Tn1 0 involves three distinct chemical steps, first-strand nicking, hairpi n formation, and hairpin resolution. This three-step mechanism makes i t possible for a single protein-active site to cleave two DNA strands of opposite polarity, as appears to be the case in this reaction. We i nfer the existence of alternating bifunctionality within the active si te with suitable modulation of substrate components between steps. DNA double-strand breaks are also made by a ''hairpin mechanism'' in V(D) J recombination, possibly reflecting the same basic constraints faced in the Tn10 system. Similarities in the basic chemical steps in Tn10 t ransposition and V(D)J recombination suggest that the V(D)J mechanism may have evolved from a bacterial transposition system.