Features of Expression of Foreign Genes in Complex Insertions in Transgenic Tobacco Plants with a Mosaic Pattern of nptII Gene Expression

The features of a mosaic pattern of the expression of the nptII selective gene, which provides plant resistance to kanamycin antibiotic, and the target gene of Serratia marcescens secretory endonuclease under control of the bidirectional MAS promoter of the mannopine synthase gene of Agrobacterium tumefaciens Ti plasmid in epiallelic lines of transgenic tobacco plants (Nu5 and Nu6) were studied. Both genes are part of a complex insertion represented by two full-size T-DNA copies and one truncated copy located between them in the reverse orientation. Transgenic tobacco lines contrastingly differ in the phenotypic expression of the nptII gene (low frequency of mosaics in the Nu5 line and high in Nu6). It was established that a decrease in the level of expression of the selective gene occurs when transgenic plants pass from hemi- to homozygous state, and it is most pronounced for the Nu6 epiallele. It was demonstrated that synthesis of aberrant sense and antisense transcripts in the region of a truncated T-DNA copy occurs in the transgenic lines. Namely these transcripts can act as triggers and trigger the inactivation of the selective nptII gene expression.