Tryptophan Biosynthesis and Molecular Genetics Biochemical and

Tryptophan is the one of the least abundant yet, in terms of energy, one of the most expensive to produce of the standard protein amino acids (Hrazdina and Jensen, 1992). The Iow leve1 of soluble tryptophan present in plants (1 to 15 pM) belies the importance of the amino acid and the pathway that produces it (Gilchrist and Kosuge, 1980). Animals, and some eubacteria, lack the ability to synthesize tryptophan and must obtain it from plant and microbial sources (Crawford, 1989; Bentley, 1990; Herrmann et al., 1992). This essential amino acid is required by animals for protein synthesis as well as for the production of other compounds, including the neurohormone serotonin and the vitamin nicotinic acid. The primary role of tryptophan biosynthesis in bacteria and fungi isto provide the tryptophan needed for protein synthesis. In contrast, plants algo use this pathway to provide precursors for synthesis of the hormone auxin, phytoalexins, glucosinolates, and both indoleand anthranilate-derived alkaloids. A few examples of these secondary products are shown in Figure 1. Tryptophan biosynthesis thus plays a direct role in regulating plant development, pathogen defense responses, and plant-insect interactions. Volatile indolics are also implicated in attracting pollinating animals. In addition to playing important ecological roles, a number of indole alkaloids have important pharmacological uses (see Kutchan, 1995, this issue). For example, the indole alkaloids vinblastine and vincristine are widely used anticancer drugs (Wiebe and Sipila, 1994). The biosynthetic pathway for tryptophan, shown in Figure 2, exhibits a striking conservation in its biochemistry across kingdoms (identical substrates are converted into the same products by homologous enzymes at each step in prokaryotes and eukaryotes) that contrasts sharply with the rich variation in regulatory controls (Hütter et al., 1986; Crawford, 1989; Rose and Last, 1994). In the following discussion of tryptophan biosynthesis, we show that this biochemical conservation was fruitfully exploited in the isolation of the Arabidopsis structural gene cDNAs for each step in the pathway by heterologous hybridization screening and complementation of fscherichia coli mutations (Rose and Last, 1994).