The ubiC gene of E. coli encodes chorismate pyruvate-lyase, an enzyme that converts chorismate into 4-hydroxybenzoate (4HB), not normally present in plants. Transgenic tobacco plant expressing ubiC targeted to the plastid accumulated 4HB as B-glucosides to 0.5% of the dry weight. The glucose was attached to either the hydroxyl or the carboxyl function of 4HB (Siebert et al, 1994; 1996).

Targeting of the UbiC protein to either the plastids or to the cytosol results in formation of 4-hydroxybenzoate in transgenic tobacco. This suggests that chorismate is present not only in the plastids but also in the cytosol of plant cells (Sommer and Heide, 1998).

Not shown in the above metabolic scheme, isochorismate is synthesized from chorismate in a reaction catalyzed by the enzyme isochorismate synthase (ICS) (also known as isochorismate mutase) [EC 5.4.99.6]. Isochorismate is involved in the synthesis of both primary and secondary metabolites. ICS has been purified to homogeneity from elicited Catharanthus roseus cell cultures (van Tegelen et al, 1999). Two isoforms of ICS with an apparent molecular mass of 64 kD were purified and characterized, and a cDNA encoding ICS was recently isolated (van Tegelen et al, 1999). The cDNA encodes a protein of 64 kD with an N-terminal chloroplast-targeting signal. The deduced amino acid sequence shares homology with bacterial ICS and also with anthranilate synthases from plants. Southern analysis indicates the existence of only one ICS gene in C. roseus (van Tegelen et al, 1999).

Siebert M, Severin K, Heide L 1994 Formation of 4-hydroxybenzoate in Escherichia coli: characterization of the ubiC gene and its encoded enzyme chorismate pyruvate-lyase. Microbiology 140: 897-904.

Siebert M, Sommer S, Li SM, Wang ZX, Severin K, Heide L 1996 Genetic engineering of plant secondary metabolism. Accumulation of 4-hydroxybenzoate glucosides as a result of the expression of the bacterial ubiC gene in tobacco. Plant Physiol. 112: 811-819.

Sommer S, Heide L 1998 Expression of bacterial chorismate pyruvate-lyase in tobacco: evidence for the presence of chorismate in the plant cytosol. Plant Cell Physiol. 39: 1240-1244.

van Tegelen LJ, Moreno PR, Croes AF, Verpoorte R, Wullems GJ 1999 Purification and cDNA cloning of isochorismate synthase from elicited cell cultures of Catharanthus roseus. Plant Physiol. 119: 705-712.