Differential inactivation and methylation of a transgene in plants by two suppressor loci containing homologous sequences

In a previous study on doubly transformed tobacco plants, we observed the unexpected inactivation in trans of T-DNA-I (encoding Kan^rNOS) following the introduction into the same genome of an unlinked copy of T-DNA-II (encoding Hyg^rOCS). This inactivation, which probably resulted from interactions between homologous regions on each T-DNA, was correlated with methylation in the nos _pro, which controlled the expression of both the nptII and nos genes. In this paper, we show that the inactivation and methylation of the nos _pro nptII gene in the presence of a suppressor T-DNA-II locus can be either complete (epistasis) or partial (cellular mosaicism). In plants showing partial suppression, the strength of the Kan^r phenotype, which apparently reflected the proportion of cells expressing the nptII gene, was inversely correlated with the degree of methylation of the nos _pro. The extent of nos _pro methylation decreased progressively in successive generations as suppressor T-DNA-II loci were crossed out. The strength of the Kan^r phenotype was improved and nos _pro methylation was less extensive in first generation Kan^r progeny obtained from outcrossing with untransformed tobacco than from self-fertilization.