Analysis of the functional domains of biosynthetic threonine deaminase by comparison of the amino acid sequences of three wild-type alleles to the amino acid sequence of biodegradative threonine deaminase

The nucleotide sequence of the gene, ilvA, for biosynthetic threonine deaminase (Tda) from Salmonella typhimurium was determined. The deduced amino acid sequence was compared with the deduced amino acid sequences of the biosynthetic Tda from Escherichia coli K-12 (ilvA) and Saccharomyces cerevisiae ( ILV1) and the biodegradative Tda from E. coli K-12 ( tdc). The comparison indicated the presence of two types of blocks of homologous amino acids. The first type of homology is in the N-terminal portion of all four isozymes of Tda and probably indicates amino acids involved in catalysis. The second type of homology is found in the C-terminal portion of the three biosynthetic isozymes and presumably is involved in either (i) the binding or interaction of the allosteric effector isoleucine with the enzyme, or (ii) subunit interactions. The sites of amino acid changes of two E. coli K-12 ilvA alleles with altered response to isoleucine are consistent with the conclusion that the C-terminal portion of biosynthetic Tda is involved in allosteric regulation.