Methylophilus methylotrophus produced viscous and non-viscous exopolysaccharides (EPS)when grown in batch culture. Both types contained glucose, galactose, mannose and an unidentified 6-desoxyhexose, and were substituted with pyruvate and acetate residues. When the organism was grown in continuous culture only the non-viscous EPS was synthesized; the rate of production was 18.5 mg h-I (g biomass)-' in methanol-limited cultures and increased by approximately 3-and 4-fold when growth was limited by oxygen or nitrogen respectively. The specific activity of methanol dehydrogenase in cell extracts was relatively low when bacteria were grown under conditions of methanol excess and increased 2-fold in carbon-limited cells, reflecting the need to 'scavenge' the small amounts of available methanol. In contrast, the specific activities of several key enzymes of the ribulose monophosphate (RuMP) pathway were greater in cells grown under conditions of nitrogen or oxygen limitation than when growth was limited by the availability of carbon, indicating the potential for increased carbon flux round the cycle when excess methanol was present in the growth medium. When methylotrophs are grown under conditions of methanol excess it is important that there is a mechanism to prevent the overproduction of formaldehyde, and we suggest that these changes in EPS production and in the specific activities of the key enzymes of the RuMP cycle are necessary for the efficient removal of this toxic metabolite of methanol. I N T R O D U C T I O NMethylophilus methylotrophus can use reduced C1 compounds as the sole source of carbon and energy. Methanol is oxidized by a quinoprotein, methanol dehydrogenase, to formaldehyde, which then can be either further oxidized to C 0 2 or assimilated (see Anthony, 1982). In this organism the KDPG (2-keto-3-deoxy-6-phosphogluconate) aldolase/transaldolase variant of the ribulose monophosphate (RuMP) pathway is responsible for the fixation of C1 compounds and also has a dissimilatory role. M . methylotrophus also contains NAD+-linked formaldehyde and formate dehydrogenases but usually only a small proportion of formaldehyde is oxidized via these enzymes in RuMP pathway methylotrophs (Anthony, 1982). The first enzyme of the RuMP pathway is hexulose-6-phosphate synthase, which catalyses the condensation of formaldehyde with ribulose monophosphate to form hexulose 6-phosphate. This is then metabolized by hexulose-6-phosphate isomerase, glucose-6-phosphate isomerase and glucose-6-phosphate dehydrogenase. The resulting 6-phosphogluconate is at a branch point and may enter the assimilatory part of the cycle or be oxidized to ribulose monophosphate and COz in a reaction catalysed by 6-phosphogluconate dehydrogenase ; two isoenzymes are present in M . methylotrophus, one is specific for NAD+, while the other uses both NAD+ and NADP+ (Beardsmore et al., 1982).Several methylotrophic bacteria produce EPS during growth on reduced C1 compounds. ForAbbreviations: EPS, exopolysaccharide; KDPG, 2-keto-3-deoxy-6-phosphogluco...

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Methylophilus methylotrophus can use methylamine as sole source of carbon and nitrogen. Measurements of the specific activity of methylamine dehydrogenase (MNDH) in bacteria grown in batch or chemostat culture showed that MNDH was induced by methylamine and repressed when methanol or NH4+ were provided as alternative carbon or nitrogen sources. The degree of repression varied with the growth conditions. Methanol dehydrogenase (MDH) was present in bacteria growtn on methylamine as sole carbon source, but the specific activity was low compared with that in bacteria grown on medium containing methanol, indicating that this enzyme is induced by methanol.