Abstract:
Methylotrophic bacteria are capable of growth on single-carbon compounds as their sole source of carbon and energy. This metabolism entails the production of formaldehyde as a central intermediate. Methylobacterium extorquens AM1 possess two pterin-linked C1 transfer pathways that are critical for methylotrophic growth. These utilize either tetrahydrofolate (H4F) or tetrahydromethanopterin (H4MPT) as C1 carriers. Through genetic and physiological analyses, as well as 14C label tracing studies, I have determined that the H4MPT-linked pathway is required for formaldehyde oxidation and detoxification. The H 4F-linked pathway is also required for methylotrophic growth, but apparently not as a second formaldehyde oxidation pathway. In order to test whether the H4F may actually play an assimilatory role, a deuterium label tracing method was devised. By examining the isotopomer distribution of serine-derived fragments I could determine whether the C1 units that entered the assimilatory serine cycle came from methylene-H4F generated directly from formaldehyde, or from the conversion of formate to methylene-H4F by the H4F-linked pathway. This technique led to the discovery that the H4F pathway functions in an assimilatory role. Furthermore, the deuterium labeling experiments were combined with 14C labeling to determine the flux through each branch of methylotrophic metabolism during the transition of cultures to and from methylotrophic growth. This has led to a dynamic picture of C1 metabolism that has elucidated an elegant metabolic loop through which M. extorquens AM1 maintains formaldehyde below toxic levels while balancing the distribution of C1 units to assimilatory and dissimilatory metabolism.
