Sequence requirements for target activity in site-specific recombination mediated by the Int protein of transposon Tn 1545

Excision and integration of Tn 1545 occur by reciprocal site-specific recombination between 6 (or 7)bp variable sequences present in the recombining attachment (atf) sites and designated overlap regions. We devised an assay for Tn1545 transposition in which derivatives containing the cis-acting transposition sequences (atfTn 1545) integrate into a target replicon when complemented in trans by the transposon-encoded integrase Int-Tn. This assay was used to determine the characteristics of the DNA sequence that influence target site selection. Characterization of several integration sites indicated that a 20 bp segment, designated attB, contains the sequences required for target activity. It also appeared that (i) the target activity depends upon the extent of homology between the 7bp segments flanking the overlap regions in attB and attTn1545, and (ii) the degree of homology between the two recombining overlap regions does not affect the level of target activity and has no influence on integration orientation.