Biosynthetic pathway of beta-methylnorleucine, an antimetabolite produced by Serratia marcescens

beta-Methylnorleucine biosynthesis was examined in Serratia marcescens using regulatory mutants of branched-chain amino acid biosynthesis. The accumulation of beta-methylnorleucine from norvaline in the wild-type strain was inhibited by the simultaneous additions of isoleucine, valine and leucine, although its accumulation in the derepressed mutant of isoleucine, valine and leucine biosynthesis was markedly increased and was not inhibited by additions of these amino acid. Accumulation of this compound was not observed in an isoleucine-valine auxotroph, although its accumulation was not affected in an isoleucine or leucine auxotroph. Transaminase B catalyzed the conversion of alpha-keto-beta-methylcaproate to beta-methylnorleucine. These results suggest that beta-methylnorleucine is formed from alpha-ketovalerate, alpha-ketoacid corresponding to norvaline, by enzymes of the isoleucine-valine biosynthetic pathway.