POSTTRANSCRIPTIONAL SILENCING OF PECTIN METHYLESTERASE GENE IN TRANSGENIC TOMATO FRUITS RESULTS FROM IMPAIRED PRE-MESSENGER-RNA PROCESSING

Transgenic tomato fruit expressing a truncated fruit pectin methyleste rase (PME) sense transgene under the control of CaMV 35S promoter show developmentally regulated co-silencing of the endogenous fruit PME an d sense transgene. This co-silencing is associated with the onset of f ruit PME gene expression. Detectable levels of PME protein and mRNA we re not present in ripening transgenic fruits. However, several smaller transgene RNA species hybridizing to the 3' end of the transgene are present in transgenic fruits, indicating degradation of sense transgen e transcript. RT-PCR analyses of nuclear RNA from wild-type and transg enic fruits, primed with oligo(dT), show the presence of spliced and u nspliced transcripts for the fruit PME and the transgene, indicating a post-transcriptional regulation of gene silencing. However, an accumu lation of unspliced fruit PME transcript was obtained in the nuclei of transgenic fruit, using the randomly primed cDNAs in PCR amplificatio n. Taken together, these results suggest that the ectopic expression o f the fruit PME sense transgene interferes with processing of the endo genous PME pre-mRNA.