Summary: Strain D-315 was isolated from a wild-type strain of as a methionine-sensitive mutant. In this mutant, methionine-mediated growth inhibition was reversed by threonine or homoserine. The homoserine dehydrogenase activity of strain D-315 was about 20% lower than that of the wild-type and was not inhibited by threonine. The methionine-sensitive mutation was located in the region on the chromosome, indicating that strain D-315 lacked homoserine dehydrogenase I, whose activity is inhibited by threonine and whose synthesis is multivalently repressed by threonine plus isoleucine. In a methionine bradytroph derived from strain D-315, homoserine dehydrogenase activity was high during growth in the absence of methionine and low during growth in the presence of excess methionine. Strain D-413, a homoserine auxotroph derived from strain D-315, had no detectable homoserine dehydrogenase activity, indicating that had homoserine dehydrogenase II, whose synthesis is controlled by methionine-mediated repression. The gene coding for homoserine dehydrogenase II was located in the region. Strain TA-191, having homoserine dehydrogenase I and lacking the other isoenzyme, was constructed by transduction. This strain was sensitive to threonine-mediated growth inhibition.


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