Strictosidine, a precursor to over 1000
indole alkaloids including the anti-
tumor drugs
vinblastine,
vincristine, and
camptothecin, is produced by the condensation of
tryptamine and
secologanin.
Strictosidine synthase, the
enzyme responsible for this condensation, is the first committed step in the
indole-alkaloid pathway. We have introduced a modified
cDNA encoding
Strictosidine synthase from Catharanthus roseus (L.) Don. (McKnight et al. 1990, Nucl.
Acids Res. 18, 4939) driven by the CaMV 35S promoter into tobacco (Nicotiana tabacum L.). Transgenic tobacco plants expressing this construct had from 3 to 22 times greater strictosidinesynthase activity than C. roseus plants. Ultrastructural immunolocalization demonstrated that
strictosidine synthase is a vacuolar
protein in C. roseus and is correctly targeted to the vacuole in transgenic tobacco. Immunoblot analysis of
strictosidine synthase showed that two distinct forms of the
enzyme were produced in transgenic tobacco plants but that only a single form was made in C. roseus. This observation indicates that the second form of the
protein is not simply a result of overexpression in tobacco, but may reflect differences in
protein processing between tobacco and C. roseus.