The Journal of Biological Chemistry

The experiments with isotopes of the group at the Carnegie Institution of Washington laboratory (1) led to the concept of the aspartic acid family of amino acids. It was shown that, in Escherichiu co.%, aspartic acid contributes carbon to form all or part of the carbon skeleton of threonine, methionine, isoleucine, lysine, and diaminopimelic acid. The intermediates in the transformation of aspartic acid to threonine have been elucidated through the work of Black and Wright (2, 3) and Watanabe, Konishi, and Shimura (4, 5). In addition, one of these intermediates, homoserine, has been established as a precursor of methionine (6) whereas threonine is an intermediate in the formation of isoleucine from aspartic acid (7). These relationships are summarized in Fig. 1. Experiments with isotopes have eliminated homoserine or threonine as possible direct precursors of diaminopimelic acid and lysine (1). Therefore, the branch point in the aspartic acid family of amino acids, which leads to diaminopimelic acid and lysine, must occur at aspartic acid itself or at aspartyl phosphate or aspartic semialdehyde. This paper will be concerned with the properties of an auxotrophic mutant that have provided evidence for placing the branch point at aspartic semialdehyde. A preliminary communication of these results has appeared (8).