Disruption of the petB-petD intergenic region in tobacco chloroplasts affects petD RNA accumulation and translation.

Translation initiation in chloroplasts is a complex process involving a variety of cis-elements and trans-acting factors. Many chloroplast mRNAs are processed products of polycistronic primary transcripts, but the functional requirement for processing is mostly enigmatic. In tobacco, the petB and petD genes, which encode subunits of the cytochrome b6/f complex, are transcribed from the psbB operon, whose primary transcript is processed into products including di- or tricistronic, but not monocistronic, petB and petD mRNAs. To begin to identify elements important for petB and/or petD translation, we generated tobacco chloroplast transformants by inserting selectable aadA marker cassette in the petB-petD intergenic region. The resulting plants required sucrose for growth, and their phenotypes depended on the orientation of the aadA cassette. When aadA was inserted in the same transcriptional orientation as the psbB operon, petB and petD mRNAs were abundantly produced but aberrant in size, and only 25% of the wild-type amount of the cytochrome b6/f complex accumulated. With the aadA cassette in the opposing orientation, however, very little petD mRNA accumulated, and the cytochrome b6/f complex was undetectable. Polysome analysis suggested that petD mRNAs in both transformants were poorly translated, indicating that the intergenic region contains essential translational elements.