RESUMO
The aspartate pathway of amino acid biosynthesis in bacteria serves as paradigm for the evolution of patterns of enzyme regulation in response to specific physiological requirements. In Bacillus species, the first step in the pathway is catalyzed by multiple forms of aspartokinase, which differ in their structure and feedback regulation. One form of aspartokinase (V-type) functions primarily during cell growth, another form (S-type) during sporulation. The V-type aspartokinase from Bacillus subtilis and Bacillus polymyxa is discussed in some detail on account of its complex pattern of regulation by the pathway endproducts lysine and threonine and its unusual subunit structure. The enzyme is composed of two dissimilar subunits, the smaller of which corresponds to the carboxyl-terminal domain of the larger subunit. The coding sequence for the subunits of Bacillus subtilis aspartokinase has recently been cloned in Escherichia coli. The study of its structure and mode of expression has revealed that the two aspartokinase subunits are encoded by in-phase overlapping genes. These unusual features of aspartokinase suggest that important aspects of the regulation of the aspartate pathway are yet to be discovered.