Formaldehyde-Detoxifying Role of theTetrahydromethanopterin-Linked Pathway in Methylobacteriumextorquens AM1

The facultative methylotroph Methylobacterium extorquens AM1 possesses two pterin-dependent pathways for C 1 transfer between formaldehyde and formate, the tetrahydrofolate (H 4 F)-linked pathway and the tetrahydromethanopterin (H 4 MPT)-linked pathway. Both pathways are required for growth on C 1 substrates; however, mutants defective for the H 4 MPT pathway reveal a unique phenotype of being inhibited by methanol during growth on multicarbon compounds such as succinate. It has been previously proposed that this methanol-sensitive phenotype is due to the inability to effectively detoxify formaldehyde produced from methanol. Here we present a comparative physiological characterization of four mutants defective in the H 4 MPT pathway and place them into three different phenotypic classes that are concordant with the biochemical roles of the respective enzymes. We demonstrate that the analogous H 4 F pathway present in M. extorquens AM1 cannot fulfill the formaldehyde detoxification function, while a heterologously expressed pathway linked to glutathione and NAD + can successfully substitute for the H 4 MPT pathway. Additionally, null mutants were generated in genes previously thought to be essential, indicating that the H 4 MPT pathway is not absolutely required during growth on multicarbon compounds. These results define the role of the H 4 MPT pathway as the primary formaldehyde oxidation and detoxification pathway in M. extorquens AM1.