DNA methylation and expression of NPT II in transgenic petunias and progeny

The expression and inheritance of the NPT II (neomycin phosphotransferase II) gene was studied in four transgenic petunia (Petunia hybrida Vilm.) plants and their progeny. The four transgenic plants, each of which had more than one site of insertion, were different from each other in the level of foreign gene expression. Transmission of one or more NPT II alleles to progeny as deteceted by DNA hybridization did not lead to consistant or predictable patterns of NPT II expression. All transgenic plants and their progeny displaying NPT II enzyme activity contained unmethylated SstII (methylation-sensitive restriction enzyme) sites in the nopaline synthase (NOS) promoter (controlling NPT II gene transcription); whereas, 13 of 17 plants which contained the NPT II gene and which showed no NPT II activity had methylated SstII sites. Two progeny of 1 transgenic plant appeared to have some unmethylated SstII sites, but no NPT II enzyme activity was found in leaf tissue samples. DNA methylation of the SstII site in the NOS promoter is strongly correlated with a decrease in NPT II gene expression in transgenic petunia plants and their progeny. However, DNA methylation alone could not account for the variability seen in NPT II gene expression.