Characterisation of the tryptophan synthase alpha subunit in maize.
BACKGROUND
In bacteria, such as Salmonella typhimurium, tryptophan is synthesized from indole-3-glycerole phosphate (IGP) by a tryptophan synthase alphabetabetaalpha heterotetramer. Plants have evolved multiple alpha (TSA) and beta (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 alpha-reaction (cleavage of IGP), and forms a tryptophan synthase complex with ZmTSB1 in vitro. The catalytic efficiency of the alpha-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 alpha-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.


CONCLUSIONS
It was demonstrated in vivo and in vitro that maize forms a tryptophan synthase complex and ZmTSA functions as alpha-subunit in this complex.
Kriechbaumer
Verena
V
Lehrstuhl für Genetik, Technische Universität München, D-85350 Freising, Germany. verena.kriechbaumer@wzw.tum.de
Weigang
Linda
L
Fiesselmann
Andreas
A
Letzel
Thomas
T
Frey
Monika
M
Gierl
Alfons
A
Glawischnig
Erich
E
D000595:Amino Acid Sequence; D002457:Cell Extracts; D002850:Chromatography, Gel; D049452:Green Fluorescent Proteins / Q000378:metabolism; D007700:Kinetics; D008969:Molecular Sequence Data; D010455:Peptides / Q000737:chemistry; D018515:Plant Leaves / Q000201:enzymology / Q000378:metabolism; D010940:Plant Proteins / Q000737:chemistry / Q000378:metabolism; D021122:Protein Subunits / Q000378:metabolism*; D021381:Protein Transport; D011994:Recombinant Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013347:Subcellular Fractions / Q000201:enzymology; D014367:Tryptophan Synthase / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D003313:Zea mays / Q000201:enzymology*
D002457:Cell Extracts; D010455:Peptides; D010940:Plant Proteins; D021122:Protein Subunits; D011994:Recombinant Proteins; C485184:enhanced green fluorescent protein; D049452:Green Fluorescent Proteins; D014367:Tryptophan Synthase