Characterization of Two Methanopterin Biosynthesis Mutants of Methylobacterium extorquens AM1 by Use of a Tetrahydromethanopterin Bioassay

An enzymatic assay was developed to measure tetrahydromethanopterin (H 4 MPT) levels in wild-type and mutant cells of Methylobacterium extorquens AM1. H 4 MPT was detectable in wild-type cells but not in strains with a mutation of either the orf4 or the dmrA gene, suggesting a role for these two genes in H 4 MPT biosynthesis. The protein encoded by orf4 catalyzed the reaction of ribofuranosylaminobenzene 5′-phosphate synthase, the first committed step of H 4 MPT biosynthesis. These results provide the first biochemical evidence for H 4 MPT biosynthesis genes in bacteria.