Nocardia sp. 239, a facultative methanol utilizer with the ribulose monophosphate pathway of formaldehyde fixation

Abstract

A study was made of the enzymology of methanol metabolism by a Nocardia sp. The organism, previously known as Streptomyces sp. 239 is a facultative methylotroph, which assimilates methanol via the ribulose monophosphate (RuMP) pathway of formaldehyde fixation. This was evident from the high levels of hexulose phosphate synthase in methanol-grown cells and from the high apparent growth yields of methanol-limited chemostat cultures (0.51–0.55 g cells/g methanol). Methanol is probably oxidized by a quinoprotein type of dehydrogenase as was indicated by the excretion of pyrrolo-quinoline quinone (PQQ) by methanol-grown cells. However, the enzyme is not a normal classical methanol dehydrogenase (EC 1.1.99.8). The fate of formaldehyde in the dissimilation of methanol by the organism is obscure. A cyclic oxidation of formaldehyde as found in all other methanol utilizers with the RuMP pathway, appears unlikely since the activities of glucose-6-phosphate dehydrogenase and of 6-phosphogluconate dehydrogenase were rather low. Furthermore cells grown in carbon-limited chemostat cultures on mixtures of ethanol and formaldehyde did not contain hexulose phosphate synthase activity. Although methanol-grown cells showed a low formate-oxidizing capacity, neither dye-linked nor NAD-linked formate dehydrogenase could be detected in cell-free extracts. During growth of Nocardia sp. 239 in batch culture on a mixture of methanol and ethanol diauxic growth was observed. Oxidation of methanol and excretion of PQQ preceeded the synthesis of hexulose phosphate synthase indicating that the synthesis of dissimilatory and assimilatory enzymes is not coordinately controlled. It is concluded that in Nocardia sp. 239 the oxidation of methanol proceeds via a mechanism which must be different from those encountered so far in Gram-negative methylotrophs.