Expression instability of the marker nptII gene in transgenic tobacco plants

Instability of the marker gene far neomycin phosphotransferase II (nprII) e xpression was studied in transgenic plants of tobacco (Nicotiana tabacum L. , line SR1). In 90 out of the 102 independently produced transformants, the transgene in the first self-pollinated generation was inherited according to the Mendel laws, whereas six plants deviated from this segregation patte rn. Two plants did not express nptII at all, and the frequency of reversion in their progeny was 1 x 10(-3). Increasing the number of T-DNA copies in the genome of hybrid plants sometimes resulted in deviation from the mendel ian segregation. Introducing the duplicated beta-glucuronidase gene sequenc es into T-DNA inserts correlated with nptII instability. Transgenic plants harboring inactivated nptII present a promising model to study the molecula r-genetic mechanisms of transgene silencing.