Analysis of extrachromosomal Ac/Ds transposable elements

The mechanism of transposition of the maize Ac/Ds elements is not well unde rstood. The true transposition intermediates are not known and it has not b een possible to distinguish between excision models involving 8-bp staggere d cuts or 1-bp staggered cuts followed by hairpin formation, in this work, we have analyzed extrachromosomal excision produces to gain insight into th e excision mechanism. Plasmid rescue was used to demonstrate that Ds excisi on is associated with the formation of circular molecules. Tn addition, we present evidence for the formation of linear extrachromosomal species durin g Ds excision. Sequences found at the termini of circular and linear elemen ts showed a broad range of nucleotide additions or deletions, suggesting th at these species are not true intermediates. Additional nucleotides adjacen t to the termini in extrachromosomal elements were compared to the sequence of the original donor site. This analysis showed that: (1) the first nucle otide adjacent to the transposon end was significantly more similar to the first nucleotide nanking the element in the donor site than to a random seq uence and (2) the second and farther nucleotides did not resemble the donor site. The implications of these findings for excision models are discussed .