d-Amino Acids as Inducers of l-Alanine Dehydrogenase in Bacillus subtilis

Abstract l-Alanine dehydrogenase in Bacillus subtilis is inducible by its substrate, l-alanine. In addition to l-alanine, 10 other l-amino acids as well as d-alanine and 11 other d-amino acids are also inducers. The physiological role of the enzyme is to catabolize l-alanine to pyruvate and ammonia. The majority of the l-amino acid inducers can be converted to l-alanine by l-alanine transaminase. Similarly, the majority of the d-amino acid inducers can be converted to d-alanine by d-alanine transaminase. l- and d-Alanine, in turn, are interconvertible by alanine racemase. In a mutant deficient in alanine racemase, neither l-alanine nor any of the other l-amino acid inducers can induce l-alanine dehydrogenase, whereas the enzyme is still inducible by d-alanine and the other d-amino acid inducers. These results show that the l-amino acids induce l-alanine dehydrogenase only if they can be converted first to l-alanine and then to d-alanine. The pattern of control observed illustrates a new form of end product regulation, whereby d-alanine regulates its own biosynthesis via alanine racemase by the induction of l-alanine dehydrogenase. l-Alanine dehydrogenase catabolizes l-alanine, and thereby limits the amount of l-alanine available to alanine racemase for the synthesis of d-alanine.