Biosynthesis of the Branched-Chain Amino Acids

Branched-chain amino acid biosynthesis has been most thoroughly characterized in Bacillus subtilis; hence, most of the information in this chapter is from studies of B. subtilis. The novel mechanisms for biosynthesis in other gram-positive genera are also described. In addition, the organization of the genes responsible for isoleucine, valine, and leucine biosynthesis and proposed regulatory mechanisms for expression of those genes is elucidated and compared with those for the corresponding genes in the gram-negative bacterium Escherichia coli. The chapter illustrates the major pathways for the synthesis of the branched-chain amino acids in vegetatively growing B. subtilis cells. In Bacillus spp., some of the intermediates in branched-chain amino acid biosynthesis, specifically the branched-chain keto acids, are the immediate precursors of synthesis of the branched-chain fatty acids that are an integral part of the cell membrane. The chapter also talks about the regulation of ilv and leu gene expression. A brief overview of what occurs in E. coli is presented first, and then what occurs in B. subtilis is described. The genetic organization and regulation of the ilv and leu genes are significantly different in grampositive and gram-negative bacteria. As in E. coli, B. subtilis cells increase expression of the ilv and leu genes when there is a need for the branched-chain amino acids. Much more investigation is needed before the complete story of branched-chain amino acid biosynthesis in the gram-positive bacteria is known.