TAM3 IN ANTIRRHINUM-MAJUS IS EXCEPTIONAL TRANSPOSON IN RESISTANT TO ALTERATION BY ABORTIVE GAP REPAIR - IDENTIFICATION OF NESTED TRANSPOSONS

Most transposon families consist of heterogeneous copies with varying sizes. In contrast, the Tam3 copies in Antirrhinum majus are known to have exceptionally conserved structures of uniform size. Gap repair ha s been reported to be involved in the structural alteration of copies from several transposon families. In this study, we have asked whether or not gap repair has affected Tam3 copies. Five Tam3 copies carrying aberrant sequences were selected from 40 independent Tam3 clones and their sequences were analyzed. Two of the five copies contain insertio ns in the Tam3 sequence. These two insertions, designated Tam356 and T am661, are typical transposon-like sequences, which have terminal inve rted repeats and cause target site duplication. These nested transposo ns were obviously associated with transpositional events, and did not originate from the gap-repair process. The remaining three copies had lost large parts of the Tam3 sequence. We could not find any relations hip between the deletions of Tam3 sequence in the three copies and gap repair. PCR analysis of a Tam3 excision site in the nivea(recurrence: Tam3) mutant also showed that most of the repair events after the Tam3 excision involved end-joining. In addition to the results obtained he re, among the other clones isolated, we could not find any of the inte rnally deleted copies that comprise a major part of other transposon f amilies. All of these data suggest that some feature of the Tam3 struc ture suppresses the structural alterations that are otherwise generate d during the gap repair process.