The tryptophan synthase a subunit catalyzes the conversion of indole-3
-glycerolphosphate to indole, the penultimate reaction in the biosynth
esis of the essential amino acid tryptophan. A cDNA encoding Arabidops
is thaliana tryptophan synthase alpha(TSA1) was isolated by complement
ation of an Escherichia coli Delta trpA mutation and by polymerase cha
in reaction amplification from a cDNA library using degenerate primers
. A TSA1 genomic clone was also isolated and 5 kb of the DNA sequence
determined. A single sequence in the Arabidopsis genome with homology
to the TSA1 cDNA was detected by high-stringency genomic Southern blot
hybridization. In contrast under hybridization conditions of reduced
stringency, one or two additional homologous sequences were observed.
A 1.4 kb transcript was detected in wild-type RNA with the TSA1 cDNA a
s a probe. Several lines of evidence, including immunoaffinity chromat
ography, suggest that the active A. thaliana tryptophan synthase enzym
e consists of a heterosubunit complex, presumably analogous to the pro
karyotic alpha(2) beta(2) complex. Immunoblot analysis indicated that
the plant a and beta subunits are present throughout development.