Regulation of activator/dissociation transposition by replication and DNA methylation

In maize the transposable elements Activator/Dissociation (Ac/Ds) transpose shortly after replication from one of the two resulting chromatids ("chrom atid selectivity"). A model has been suggested that explains this phenomeno n as a consequence of different affinity for Ac transposase binding to holo -, hemi-, and unmethylated transposon ends. Here we demonstrate that in pet unia cells a holomethylated Ds in unable to excise from a nonreplicating ve ctor and that replication restores excision. A Ds element hemi-methylated o n one DNA strand transposes in the absence of replication, whereas hemi-met hylation of the complementary strand causes a >6.3-fold inhibition of Ds ex cision. Consistently in the active hemi-methylated state. the Ds ends have a high binding affinity for the transposase, whereas binding to inactive en ds is strongly reduced. These results provide strong evidence for tile abov e-mentioned model. Moreover, in the absence of DNA methylation, replication enhances Ds transposition in petunia protoplasts >8-fold and promotes form ation of a predominant excision footprint. Accordingly, replication also ha s a methylation-independent regulatory effect on transposition.