Aromatic Amino Acid Metabolism in Bacillus subtilis

Most proteins of Bacillus subtilis contain the three aromatic amino acids--phenylalanine, tyrosine, and tryptophan--all of which are synthesized from a common precursor, chorismate. The known and putative B. subtilis genes of aromatic amino acid metabolism, their location and presumed organization in operons, and their likely enzyme products or function are summarized in this chapter. Important distinctions between B. subtilis and Escherichia coli with regard to aromatic amino acid metabolism are the existence of a multifunctional aromatic supraoperon in B. subtilis and this organism's use of cross-pathway regulation of gene expression. Trp RNA-binding attenuation protein (TRAP) activation is reduced when the hydrogen bond between Tht52 and the catboxyl group of tryptophan is prevented. The affinity of TRAP for tryptophan increases in the presence of bound RNA. In B. subtilis, the biosynthetic pathways for folate and tryptophan are highly interconnected. First, both pathways use chorismate and glutamine to synthesize the p-aminobenzoic acid and o-aminobenzoic acid precursors of folate and tryptophan, respectively. Next, the TrpG polypeptide functions as the glutamine amidotransferase component of two enzyme complexes. Further, the mtrAB operon encodes TRAP (mtrB) and GTP cyclohydrolase I (mrrA). Finally, TRAP negatively regulates both biosynthetic pathways in response to tryptophan. The chapter talks about tRNA trp regulation of trp gene expression, a gene coding for a putative tryptophan transport protein, and homologous polypeptides in some other bacterial species.