SWITCHING FROM CUT-AND-PASTE TO REPLICATIVE TN7 TRANSPOSITION

The bacterial transposon Tn7 usually moves through a cut-and-paste mec hanism whereby the transposon is excised from a donor site and joined to a target site io form a simple insertion. The transposon was conver ted to a replicative element that generated plasmid fusions in vitro a nd cointegrate products in vivo. This switch was a consequence of the separation of 5'- and 3'-end processing reactions of Tn7 transposition as demonstrated by the consequences of a single amino acid alteration in an element-encoded protein essential for normal cut-and-paste tran sposition. The mutation specifically blocked cleavage of the 5' strand at each transposon end without disturbing the breakage and joining or ! the 3' strand, producing a fusion (the Shapiro Intermediate) that re sulted in replicative transposition. The ability of Tn7 recombination products to serve as substrates for both the limited gap repair requir ed to complete cut-and-paste transposition and the extensive DNA repli cation involved in cointegrate formation suggests a remarkable plastic ity in Tn7's recruitment of host repair and replication functions.