DIRECT EVIDENCE FOR RAPID DEGRADATION OF BACILLUS-THURINGIENSIS TOXINMESSENGER-RNA AS A CAUSE OF POOR EXPRESSION IN PLANTS

It is well established that the expression of Bacillus thusingiensis ( B.t.) toxin genes in higher plants is severely limited at the mRNA, le vel, but the cause remains controversial. Elucidating whether mRNA acc umulation is limited transcriptionally or posttranscriptionally could contribute to effective gene design as well as provide insights about endogenous plant gene-expression mechanisms. To resolve this controver sy, we compared the expression of an A/U-rich wild-type crylA(c) gene and a G/C-rich synthetic crylA(c) B.t.-toxin gene under the control of identical 5' and 3' flanking sequences. Transcriptional activities of the genes were equal as determined by nuclear run-on transcription as says. In contrast, mRNA half-life measurements demonstrated directly t hat the wildtype transcript was markedly less stable than that encoded by the synthetic gene. Sequences that limit mRNA accumulation were lo cated at more than one site within the coding region, and some appeare d to be recognized in Arabidopsis but not in tobacco (Nicotiana tabacu m). These results support previous observations that some A/U-rich seq uences can contribute to mRNA instability in plants. Our studies furth er indicate that some of these sequences may be differentially recogni zed in tobacco cells and Arabidopsis.