THE MTRAB OPERON OF BACILLUS-SUBTILIS ENCODES GTP CYCLOHYDROLASE-I (MTRA), AN ENZYME INVOLVED IN FOLIC-ACID BIOSYNTHESIS, AND MTRB, A REGULATOR OF TRYPTOPHAN BIOSYNTHESIS

mtrA of Bacillus subtilis was shown to be the structural gene for GTP cyclohydrolase I, an enzyme essential for folic acid biosynthesis. mtr A is the first gene in a bicistronic operon that includes mtrB, a gene involved in transcriptional attenuation control of the trp genes. mtr A of B. subtilis encodes a 20-kDa polypeptide that is 50% identical to rat GTP cyclohydrolase I. Increased GTP cyclohydrolase I activity was readily detected in crude extracts of B. subtilis and Escherichia col i in which MtrA was overproduced. Biochemical evidence indicating that MtrA catalyzes dihydroneopterin triphosphate and formic acid formatio n from guanosine triphosphate is presented. It was also shown that mtr B of B. subtilis encodes a 6-kDa polypeptide. Expression of mtrB is su fficient for transcriptional attenuation control of the B. subtilis tr p gene cluster in Escherichia coli. Known interrelationships between g enes involved in folic acid and aromatic amino acid biosynthesis in B. subtilis are described.