Internal homologies in the two aspartokinase-homoserine dehydrogenases of Escherichia coli K-12.

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Abstract
In E. coli, aspartokinase I-homoserine dehydrogenase I and aspartokinase II-homoserine dehydrogenase II are 2 bifunctional proteins, derived from a common ancestor, that catalyze the 1st and 3rd reactions of the common pathway leading to threonine and methionine. An extensive amino acid sequence comparison of the molecules reveals 2 main features on each of them: 2 segments, each of .apprx. 130 amino acids, covering the first 1/3 of the polypeptide chain, are similar to each other and 2 segments, each of .apprx. 250 amino acids and covering the COOH-terminal 500 amino acids, also present a significant homology. These 2 regions may have evolved independently of each other by a process of gene duplication and fusion previous to the appearance of an ancestral aspartokinase-homoserine dehydrogenase molecule.