Isochorismate synthase cDNA isolation from cell cultures of Catharanthus roseus (L.)G.Don.

Abstract

Biosynthetic routes leading to plant secondary metabolism start from products of primary pathways. This makes the question, how primary and secondary metabolism are mutually regulated at the molecular level, an intriguing one. A major pathway in primary and secondary metabolism is the shikimate pathway (Herrmann, 1995). This pathway provides chorismate that is used not only for the synthesis of the aromatic amino acids, but also serves as precursor for a wide variety of aromatic substances (Herrmann, 1995; Weaver and Herrmann, 1997). In addition, chorismate is the starting point for a biosynthetic pathway leading to phylloquinones (vitamin K1) and anthraquinones (Poulsen, Verpoorte, 1991). The conversion of chorismate into isochorismate is the first committed step in this pathway. This step is catalysed by the enzyme isochorismate synthase (ICS) (Poulsen, Verpoorte, 1991). Since its substrate, chorismate, plays a dual role in the synthesis of compounds derived from this pathway and their distribution over the various sub-pathways, it is expected that this enzyme is tightly regulated. Model systems to study this regulation are elicited cell cultures of Catharanthus roseus, since they provide an example of the partitioning of chorismate. Elicitation of C. roseus with a fungal extract induces not only several enzymes of the indole alkaloid biosynthetic pathway (Pasquali et al, 1992), but also ICS (Moreno et al, 1994). Knowledge on the expression kinetics and on the biochemical characteristics of the enzymes that compete for chorismate (ICS, chorismate mutase, and anthranilate synthase) may help to understand the regulation of the distribution of this precursor over the various pathways.