PROMOTER-DEPENDENT TRANS-INACTIVATION IN TRANSGENIC TOBACCO PLANTS - KINETIC ASPECTS OF GENE SILENCING AND GENE REACTIVATION

The kinetics of gene silencing and gene reactivation were followed in a transgenic plant carrying a ''silencing locus'' which inhibit, in tr ans, the expression of 19S and 35S transgenes irrespective of the sequ ence being expressed from these promoters and irrespective of their lo cation within the genome, and which induces their methylation de novo. Results of retransformation of plants carrying the 'silencing locus'' show that the transient expression of extrachromosomal copies of ''ta rget'' genes are not affected whereas the expression of ''target'' gen es is not detected in stable transformants. Results of sexual crosses between plants carrying the ''silencing locus'' and plants carrying 't arget'' genes show that trans-inactivation starts to occur rapidly dur ing the formation of hybrid seeds. Altogether, these results suggest t hat, although a complete inactivation may require few weeks, the proce ss of trans-inactivation begins soon after the meeting of the 'target' gene and the ''silencing locus''. Conversely, segregation of the ''si lencing locus'' and the ''target locus'' in the progeny leads to a pro gressive rather than an immediate re-expression of the silenced gene a t the ''target locus''. Four months after the elimination of the ''sil encing locus'', the expression of the 'target'' gene ranged between 4 % and 72 % of control plants that have never carried the 'silencing lo cus''. This silent state correlates with the maintenance of the methyl ation of the 'target'' gene. These results suggest that the silent sta te which is imprinted onto ''target' transgenes is somatically stable in the presence of the 'silencing locus'' whereas it is metastable and progressively lost after the elimination of the ''silencing locus''.