.In the course of studies on "negative feedback control" rnechanisnls caused by threonine and nlethionine in L-homoserine fermentation with Micrococcus glutarnicus 534:-Co147 culture, L-homoserine dehydrogenase was first found to occur in the culture, and effects of various amino acids on the formation and action of the enzyme wcre then invcstigated. The rcsult was that L-homoserine dehydrogenase of the culture was repressed by methionine on the one hand, and inhibited by threonine on the other. .a-Aspartokinase was found not to be reprcsscd by methionine. From these findings, the sites at which the "negative feedback control" mechanisms work were discussed.

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The first evidence that homoserine was a precursor of threonine was obtained from investigations into a mutant of Neurospora crassa requiring both methionine and threonine for growth; homoserine replaced both methionine and threonine for the growth of this mutant (1). In 1951 homoserine and threonine were found by FLING and HoRowITz (2) to accumulate in cultures of a methionineless mutant of N. crassa. Homoserine has thereafter come to be regarded as an important precursor for the biosynthesis of threonine, methionine (3, 4), and isoleucine (3, 4).Attempts were therefore made to explore microorganisms capable of producing an appreciable amount of homoserine, and as a result two strains of threonine-requiring mutants of Micrococcus glutamicus (534-Co147 and 534-185) were found to answer the purpose.Quantitative determinations of L-homoserine in culture broth were made by turbidimetric bioassay with Escherichia coli 177-184 which was kindly furnished by Dr. B. D. DAVIS (5, 6). The mutant 177-184, which responds either to L-homoserine or to L-threonine plus n, L-methionine, was instrumental in L-homoserine determination, since it failed to respond to n, Lmethionine in the absence of L-threonine and since the latter was scarcely present in our assays.A natural medium divised for the production of L-homoserine consisted of 10 % glucose, 1 %, NZ-amine, 0.5 % yeast extract, 2 % (NH4)2SO4, 2 % CaCO3, 0.1% K2HPO4, and 0.03% MgSO4.7aq.Two hundred and fifty ml-Erlenmeyer flasks containing 25 ml of the inoculated medium were shaken at 28°C on a rotary shaker, operated at 220 r.p.m. and describing a circle of 5.0 cm dia. The yields of L-homoserine was normally about 7 mg/ml, where L-lysine production was in amounts equal to, or larger than L-homoserine.A chemically defined medium for homoserine production was also devised, and found to give the yields of about 14 mg/ml of L-homoserine.The medium contained 10% glucose, 400 r/ml of L-threonine, 5 to 10 r/L biotin, 2% (NH4)2SO4, 2% CaCO3, 0.1% K2HPO4, and 0.03% MgSO4.7aq.L-Lysine yields were about 9 mg/ml. The chemical changes in L-homoserine fermentation on the synthetic medium are shown in Fig. 1. 193

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