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