RNA-MEDIATED RNA DEGRADATION AND CHALCONE SYNTHASE-A SILENCING IN PETUNIA

Transgenic Petunia plants with a chsA coding sequence under the contro l of a 35S promoter sometimes lose endogene and transgene chalcone syn thase activity and purple flower pigment through posttranscriptional c hsA RNA degradation. In these plants, shorter poly(A)(+) and poly(A)(- ) chsA RNAs are found, and a 3' end-specific RNA fragment from the end ogene is more resistant to degradation. The termini of this RNA fragme nt are located in a region of complementarity between the chsA 3' codi ng region and its 3' untranslated region. Equivalent chsA RNA fragment s remain in the white flower tissue of a nontransgenic Petunia variety . We present a model involving cycles of RNA-RNA pairing between compl ementary sequences followed by endonucleolytic RNA cleavages to descri be how RNA degradation is likely to be promoted.