Selective recognition of pyrimidine motif triplexes by a protein encoded by the bacterial transposon Tn7

The bacterial transposon Tn7 is distinguished among mobile genetic elements by its targeting abilities. Recently, we reported that Tn7 is able to sele ctively insert adjacent to triple-helical DNA. The binding of TnsC, a Tn7-e ncoded protein, to the tripler DNA target leads to the specific transpositi on of Tn7 adjacent to both inter- and intramolecular pyrimidine motif tripl exes. Here, we further probe how Tn7 targets tripler DNA. We report that Tn sC discriminates between different types of triplexes, showing binding pref erence for pyrimidine but not for purine motif intermolecular tripler DNA. The binding preferences of TnsC and the Tn7 insertion profiles were obtaine d using psoralenated, triplex-forming oligonucleotides annealed to plasmid DNAs. Although the presence of psoralen is not required for targeting nor i s it alone able to attract TnsC, we show that the location of psoralen with in the pyrimidine motif tripler does alter the position of Tn7 insertion re lative to the triplex. Comparison between the tripler-targeting pathway and the highly site-specific targeting pathway mediated by the binding of the Tn7-encoded protein, TnsD, to the unique site attTn7, suggests that similar structural features within each target DNA are recognized by TnsC, leading to site-specific transposition. This work demonstrates that a prokaryotic protein involved in the targeting and regulation of Tn7 translocation, TnsC , can selectively recognize pyrimidine motif triplexes. (C) 2001 Academic P ress.