REGULATION OF BRANCHED BIOSYNTHETIC PATHWAYS IN BACTERIA

Some complex interdependent regulatory interactions of end-product metabolites on the synthesis and activities of various enzymes involved in the biosynthesis of these end products are described. Analysis of the experimental data with aspartokinase, homoserine dehydrogenase, and threonine deaminase in a variety of microorganisms reveals that several distinct overall control patterns can be identified. In certain bacteria concerted action of two end products controls the early enzymes; in others, the existence of multiple forms of the same enzyme, each regulated by one metabolite, appears to be the principal mode of control. Compensatory effects of more than one modifier of opposing influence as well as the dual regulatory role of a single end product for two interconnecting pathways underline other alternative control characteristics. Each pattern appears adequate for a normal flow of metabolites from aspartate to various amino acid end products, and each scheme is peculiarly suited to the physiology and metabolism of the organism. The examples show that common enzymatic steps and similar control patterns do not necessarily reflect evolutionary affinities; diversity in the control mechanisms also a priori does not indicate independent phylogenetic relationships.