Coordinated regulation of two indole alkaloid biosynthetic genes from Catharanthus roseus by auxin and elicitors.

Catharanthus roseus (periwinkle) produces a wide range of terpenoid indole alkaloids, including several pharmaceutically important compounds, from the intermediate strictosidine. The complete mRNA sequence for the enzyme strictosidine synthase (SSS) was determined. Comparison of the primary structure of the encoded protein with the amino-terminal sequence of purified SSS indicated the presence of a signal peptide of 31 amino acids in the putative primary translation product. SSS is encoded by a single-copy gene indicating that isoenzymes reported by others are formed post-translationally from a single precursor. The sss gene and the tryptophan decarboxylase gene (tdc), encoding another enzyme essential for indole alkaloid biosynthesis, are coordinately regulated. In plants steady-state mRNA levels are highest in roots. In cell suspension cultures the genes are rapidly down-regulated by auxin. In contrast, both genes are strongly induced by fungal elicitors such as Pythium aphanidermatum culture filtrate or yeast extract. Induction is a rapid, transcriptional event occurring independent of de novo protein synthesis. These results show that a first important regulatory step in the complex process leading to indole alkaloid accumulation in C. roseus suspension cells is transcription of the biosynthetic genes.