Structures and expression patterns of two tropinone reductase genes from Hyoscyamus niger

In the biosynthesis of tropane alkaloids, two tropinone reductases (TRs) catalyze reduction of tropinone to different stereoisomers, tropine and pseudotropine. Two TRs from Hyoscyamus niger have 64% of identical amino acids and hence a common evolutionary origin. In this study, genomic clones of TRs were isolated from H. niger. Their sequence comparison showed that although they have the same exon/intron organization, sequence similarity was restricted to the coding regions. In H. niger transgenic hairy roots, both TR promoters activated transcription of the reporter genes in endodermis and pericycle of the roots. A quantitative reporter assay and a nuclear run-on experiment indicated that the two genes are transcribed at a similar rate. The results indicate that although different activity levels have been observed for the TR enzymes in the H. niger root, the TR genes per se conserve similar tissue-specific expression pattern and transcriptional rate.