Enzymzyic znd Molecular Genetic Studies of Serine Production by C_1-Microorganisms

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The production process of L-serine from methanol and glycine has been developed using a methylotroph with serine pathway. We screened a high producer, Hypgomicrobium methyovorum, which is an obligate methylotroph. A glycine-resistant mutant GM2 showed improved serine production (34 mg/ml). Furthermore other methylotrophic bacteria were examined for their ability to produce L-serine from methanol and glycine in a resting cell reacton. Among strains exhibiting L-serine productivity, the strain MN43 WAS found to exhibit the highest productiveity. Under optimized conditions usng this bacterium 71 mg/ml L-serine was produced. The high L-serine degrading activity of this bacterium was entirely suppressed by adding 1 mM CdCl_2, resulting in an enhanced converison ratio of glycine to L-serine (ca. 100% molar conversion). The GM2 strain was found to have elevated activities of methanol dehvdrogenase and serine hydroxymethyltransferase (SHMT). Since thers has so for been little information on the systematic characterization of enzymes of serine pathway in methylotroph, not only the adove two enzymes but also other three enzymes, serineglyoxylate aminotransferase, hydroxypyruvate reductase, glycerate kinase, were purified to hmogeneity. The gene of SHMT was cloned and its deduced amino acid sequence had homology to those of Escherichia coli (55%) and rabbit liver (44%), whereas the enzyme of the bacterinum was immunochemically distinguishadble from those of microorganisms other than Hyphomicrobium strains and mammalian livers.