Nucleic acid sequences, vectors, recombinant host cells, transgenic plants, and methods for their preparation are disclosed. The nucleic acid sequences encode threonine deaminase proteins that catalyze the conversion of threonine to .alpha.-ketobutyrate. One or both of the amino acids at positions 447 and 481 of the encoded proteins can be selected from groups of particular amino acids. For example, the amino acid at position 447 can be alanine, isoleucine, valine, phenylalanine, tryptophan, or methionine. The amino acid at position 481 can be alanine, isoleucine, proline, phenylalanine, tryptophan, or methionine.