Plants contain two distinct classes of functional tryptophan synthase beta proteins

Tryptophan synthase beta-subunits (TSBs) catalyze the last step in tryptophan biosynthesis, i.e. the condensation of indole and serine yielding tryptophan. In microorganisms two subfamilies of TSBs (here designated as type 1 and type 2) are known, which are only distantly related. Surprisingly, in all genomes of multicellular plants analyzed genes encoding both types are present. While type 1 enzymes are well established as components of tryptophan synthase complexes, type 2 enzymes in plants have not yet been characterized. Tissue specific expression of the TSB genes from Arabidopsis thaliana was analyzed. While AtTSB1 is the predominantly expressed isoform in vegetative tissues, AtTSB1 and AtTSBtype2 reach similar transcript levels in seeds. AtTSBtype2 protein was expressed in Escherichia coli and purified. It converted indole and serine to tryptophan with a strikingly low K(m)-value for indole of ca. 74 nM. Attsbtype2 T-DNA insertion mutants showed no obvious deviation from the wild type phenotype, indicating that AtTSBtype2 function is not essential under standard growth conditions. As example for a monocot enzyme, maize TSBtype 2 was analyzed and found to be transcribed in various tissues. ZmTSBtype2 was also catalytically active and here a K(m)-value for indole of ca. 7 microM was determined. These data indicate that TSB type 2 enzymes generally are functionally expressed in plants. Their potential biological role is discussed.