EXPRESSION OF SINGLE COPIES OF A STRONGLY EXPRESSED 35S TRANSGENE CANBE SILENCED POSTTRANSCRIPTIONALLY

The bacterial UidA gene cloned between the 35S promoter with a double enhancer and the terminator sequences of the pea rbcS 9C gene was intr oduced into tobacco plants. All 11 transformants carrying the transgen e at a single locus showed silencing at each generation, but the timin g of silencing occurred at different rates in the different transforma nts. Two plants showed high levels of UidA mRNA accumulation and GUS a ctivity in young seedlings and a rapid decline of these levels during the first month of growth irrespective of the allelic state of the T-D NA. The other plants showed high levels of UidA mRNA accumulation and GUS activity in young seedlings and a slow decline of these levels dur ing the first 4 months of growth in homozygous plants, whereas these l evels decline only after 1 year of growth in hemizygous plants. Haploi ds showed the same kinetics of silencing as the homozygous plants from which they derived. Silencing correlated with a strong decrease of th e steady-state level of UidA mRNA, while the transcription of the tran sgene in the nucleus was not affected. Since haploids and hemizygous p lants derived from transformants carrying a single copy of the T-DNA s how silencing, and since all the plants express the transgene at a ver y high level before the triggering of silencing, the results suggest t hat post-transcriptional silencing occurs through a dose effect and no t through DNA-DNA interaction.