Abstract
The maize, cut-and-paste transposon
Ac/Ds
is mobile in
Saccharomyces cerevisiae
, and DNA sequences of repair products provide strong genetic evidence that hairpin intermediates form in host DNA during this transposition, similar to those formed for V(D)J coding joints in vertebrates. Both DNA strands must be broken for
Ac/Ds
to excise, suggesting that double-strand break (DSB) repair pathways should be involved in repair of excision sites. In the absence of homologous template, as expected,
Ac
excisions are repaired by nonhomologous end joining (NHEJ) that can involve microhomologies close to the broken ends. However, unlike repair of endonuclease-induced DSBs, repair of
Ac
excisions in the presence of homologous template occurs by gene conversion only about half the time, the remainder being NHEJ events. Analysis of transposition in mutant yeast suggests roles for the Mre11/Rad50 complex,
SAE2
,
NEJ1
, and the Ku complex in repair of excision sites. Separation-of-function alleles of
MRE11
suggest that its endonuclease function is more important in this repair than either its exonuclease or Rad50-binding properties. In addition, the interstrand cross-link repair gene
PSO2
plays a role in end joining hairpin ends that is not seen in repair of linearized plasmids and may be involved in positioning transposase cleavage at the transposon ends.