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Open Access Research article

Characterisation of the tryptophan synthase alpha subunit in maize

Verena Kriechbaumer1, Linda Weigang2, Andreas Fießelmann1, Thomas Letzel2, Monika Frey1, Alfons Gierl1 and Erich Glawischnig1*

Author Affiliations

1 Lehrstuhl für Genetik, Technische Universität München, D-85350 Freising, Germany

2 Analytische Forschungsgruppe des Lehrstuhls für Chemie der Biopolymere, Technische Universität München, D-85350 Freising, Germany

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BMC Plant Biology 2008, 8:44  doi:10.1186/1471-2229-8-44

Published: 22 April 2008

Abstract

Background

In bacteria, such as Salmonella typhimurium, tryptophan is synthesized from indole-3-glycerole phosphate (IGP) by a tryptophan synthase αββα heterotetramer. Plants have evolved multiple α (TSA) and β (TSB) homologs, which have probably diverged in biological function and their ability of subunit interaction. There is some evidence for a tryptophan synthase (TS) complex in Arabidopsis. On the other hand maize (Zea mays) expresses the TSA-homologs BX1 and IGL that efficiently cleave IGP, independent of interaction with TSB.

Results

In order to clarify, how tryptophan is synthesized in maize, two TSA homologs, hitherto uncharacterized ZmTSA and ZmTSAlike, were functionally analyzed. ZmTSA is localized in plastids, the major site of tryptophan biosynthesis in plants. It catalyzes the tryptophan synthase α-reaction (cleavage of IGP), and forms a tryptophan synthase complex with ZmTSB1 in vitro. The catalytic efficiency of the α-reaction is strongly enhanced upon complex formation. A 160 kD tryptophan synthase complex was partially purified from maize leaves and ZmTSA was identified as native α-subunit of this complex by mass spectrometry. ZmTSAlike, for which no in vitro activity was detected, is localized in the cytosol. ZmTSAlike, BX1, and IGL were not detectable in the native tryptophan synthase complex in leaves.

Conclusion

It was demonstrated in vivo and in vitro that maize forms a tryptophan synthase complex and ZmTSA functions as α-subunit in this complex.