Transposition of maize Ac/Ds transposable elements in the yeast Saccharomyces cerevisiae

Excision by transposons is associated with chromosome breaks; generally, ho st-cell proteins repair this damage, often introducing mutations. Many tran sposons also use host proteins in the transposition mechanism or in regulat ion(1-4). Transposition in systems lacking host factors that influence the behaviour of these transpositions is useful in determining what those facto rs are and how they work. In addition, features of transposition and regula tion intrinsic to the element itself can be determined. Maize Activator/Dis sociation (Ac/Ds) elements transpose in a wide variety of heterologous plan ts, but their characteristics in these other systems differ from those in m aize, including their response to increasing genetic dosage(5,6) and the ty pes of repair products recovered following excision(7). Two Arabidopsis tha liana mutants (iae1 and iae2) show increased Ac transposition frequencies(8 ). These mutants, and the differences mentioned above, suggest the involvem ent of host proteins in Ac/Ds activity and potential differences between th ese proteins among plant species. Here we report that Ac/Ds elements, membe rs of the hAT (hobo, Ac, Tam3) superfamily, transpose in the yeast Saccharo myces cerevisiae, an organism lacking class II ('cut and paste') transposon s. This demonstrates that plant-specific proteins are not essential for Ac/ Ds transposition. The yeast system is valuable for dissecting the Ac/Ds tra nsposition mechanism and identifying host factors that can influence transp osition and the repair of DNA damage induced by Ac/Ds. Mutations caused by Ds excision in yeast suggest formation of a DNA-hairpin intermediate, and r einsertions occur throughout the genome with a frequency similar to that in plants. The high proportion of Ac/Ds reinsertions also makes this system a n in vivo mutagenesis and reverse genetics tool in yeast and, presumably, o ther eukaryotic systems.