DNA methylation of the Ac in transgenic tobacco plants --Birgit Nelsen-Salz and Hans-Peter Doring
Transgenic tobacco plants carrying the complete Ac element from maize or an inactive deletion derivative of Ac were studied. Plants were transformed via direct DNA transfer methods or via A. tumefaciens. DNA of the transgenic plants was examined with a number of different restriction enzymes whose activity is sensitive for C methylation in their target sequence. Thirty CpGs and 26 CpXpGs were analysed up to now. In four different transgenic plants the Ac or Ds sequences remain completely unmethylated at those methylatable sites which were examined. There was one plant which showed partial methylation at the XhoI site. It is interesting to note that the PvuI site at the 5' end of the Ac sequence and at least one of three closely spaced HpaII sites at the 3' end of the Ac sequence are cleaved and thus are unmodified in complete Ac sequences as well as in the internally deleted, inactive Ac element. This is different from what has been found for Ac or Ds elements in maize (Schwartz and Dennis, Mol. Gen. Genet. 205:476-482, 1986; Schwartz, Proc. Natl. Acad. Sci. USA 86:2789-2793, 1989). We conclude that the Ac sequence is not a good target for the de novo methylation activity of the tobacco methyltransferase. If the Ac sequence used for transformation is methylated at the EcoRII sites, this preimposed methylation pattern is not recovered in the transgenic plant. The methylated EcoRII sequences are not recognized by the methyltransferase which confers maintenance of methylation patterns.

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