Premature polyadenylation at multiple sites within a Bacillus thuringiensis toxin gene-coding region

Some foreign genes introduced into plants are poorly expressed, even when transcription is controlled by a strong promoter. Perhaps the best examples of this problem are the cry genes of Bacillus thuringiensis (B.t.), which encode the insecticidal proteins commonly referred to as B.t. toxins. As a step toward overcoming such problems most effectively, we sought to elucidate the mechanisms limiting the expression of a typical B.t.-toxin gene, cryIA(c), which accumulates very little mRNA in tobacco (Nicotiana tabacum) cells. Most cell lines transformed with the cryIA(c) B.t.-toxin gene accumulate short, polyadenylated transcripts. The abundance of these transcripts can be increased by treating the cells with cycloheximide, a translation inhibitor that can stabilize many unstable transcripts. Using a series of hybridizations, reverse-transcriptase polymerase chain reactions, and RNase-H-digestion experiments, poly(A+) addition sites were identified in the B.t.-toxin-coding region corresponding to the short transcripts. A fourth polyadenylation site was identified using a chimeric gene. These results demonstrate for the first time to our knowledge that premature polyadenylation can limit the expression of a foreign gene in plants. Moreover, this work emphasizes that further study of the fundamental principles governing polyadenylation in plants will have basic as well as applied significance.