Tissue-specific expression of the beta-subunit of tryptophan synthase in Camptotheca acuminata, an indole alkaloid-producing plant.

Camptothecin is an anticancer drug produced by the monoterpene indole alkaloid pathway in Camptotheca acuminata. As part of an investigation of the camptothecin biosynthetic pathway, we have cloned and characterized a gene from C. acuminata encoding the beta-subunit of tryptophan (Trp) synthase (TSB). In C. acuminata TSB provides Trp for both protein synthesis and indole alkaloid production and therefore represents a junction between primary and secondary metabolism. TSB mRNA and protein were detected in all C. acuminata organs examined, and their abundance paralleled that of camptothecin. Within each shoot organ, TSB was most abundant in vascular tissues. Within the root, however, TSB expression was most abundant in the outer cortex. TSB has been localized to chloroplasts in Arabidopsis, but there was little expression of TSB in C. acuminata tissues where the predominant plastids were photosynthetically competent chloroplasts. Expression of the promoter from the C. acuminata TSB gene in transgenic tobacco plants paralleled expression of the native gene in C. acuminata in all organs except roots. TSB is also highly expressed in C. acuminata during early seedling development at a stage corresponding to peak accumulation of camptothecin, consistent with the idea that Trp biosynthesis and the secondary indole alkaloid pathway are coordinately regulated.