SILENCING OF BETA-1,3-GLUCANASE GENES IN TOBACCO CORRELATES WITH AN INCREASED ABUNDANCE OF RNA DEGRADATION INTERMEDIATES

Post-transcriptional gene silencing of beta-1,3 glucanase genes in the transgenic tobacco line T17 is characterised by an increased turnover and, as a consequence, reduced levels of gn1 transgene and endogenous beta-1,3 glucanase mRNAs, Here, additional gn1 RNAs, both larger and smaller than the full-length messenger, are shown to accumulate in sil enced plants of the transgenic tobacco line T17, The longer-than-full- length gn1 RNAs are the result of cryptic processing of the gn1 messen ger, The small gnl RNAs in silenced plants correspond to distal and pr oximal parts of the mature gn1 messenger. The proximal RNA products ar e intact at their 5' extremity, but terminate at different positions a t the 3'-end. The distal RNA products contain a poly(A) tail and are t runcated to various positions at the 5'-end, These observations indica te that degradation of the mature gnl transcript does not start at the 5'- or 3'-end, but rather are consistent with degradation of the gnl transcript starting with an endonucleolytic cleavage followed by inter nal exonuclease digestion. Importantly, the truncated products are mor e abundant in silenced plants than in expressing plants, This suggests , together with the previously reported silencing-related increased gn l mRNA turnover and the similar rates of gnl transcription in silenced and expressing T17 plants, that the predominant decay route for the g nl transcripts differs between silenced and expressing conditions.