Abstract

In maize the transposable elements Activator/Dissociation (Ac/Ds) transpose shortly after replication from one of the two resulting chromatids ("chromatid selectivity"). A model was suggested that explains this phenomenon as a consequence of different Ac transposase binding to holo-, hemi- and unmethylated transposon ends (Wang et al., 1996). It assumes that before replication the element is holomethylated and does not transpose because TPase can not bind to the transposon ends. Shortly after replication one of the two differentially hemimethylated daughter transposons should become transposition competent. However, DNA methylation-mediated replication dependence does not completely explain the behavior of Ac/Ds transposition. In several studies performed in monocot hosts, it was found that in a transient assay Ac/Ds element excision from extrachromosomal geminivirus vectors occurs only during vector replication, although the transfected DNAs were not Cmethylated. In this work the correlation between Ds transposition, DNA replication and DNA methylation in the dicot species Petunia hybrida was studied. Ds reporter vectors harboring TYLCV (Tomato Yellow Leaf Curl Virus) geminivirus replicon sequences and replicating in transfected petunia cells were constructed. It has been shown that the transposition of a Ds element from an extrachromosomal vector in petunia cells is regulated by DNA replication in a methylation-dependent and -independent mode. Holomethylation completely inhibits Ds excision from a non-replicating plasmid, whereas Ds transposition is restored by replication. Moreover, Ds elements that are hemimethylated on one DNA strand transpose in the absence of replication, whereas methylation on the complementary DNA strand results in at least 6.3-fold reduced excision frequencies. These data strongly support the transposition model of Wang et al. (1996). Beyond that, Ds transposition is strongly promoted by replication also in the absence of methylation. It has been shown that in petunia cells, unlike monocot hosts, replication is not a prerequisite for Ds transposition, nevertheless it enhances Ds transposition by at least a factor of 7.5. Moreover, replication promotes the formation of a predominant excision footprint. Implications on the mechanism and regulation of Ac/Ds transposition are discussed.