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Lysine was excreted by Bacillus megaterium CII19 after growth in chemically defined media containing low concentrations of sulphate. Addition of more sulphate, or cysteine or methionine stopped lysine formation; instead ornithine was excreted. All the enzymes of the pathway leading from aspartate to lysine were assayed in extracts prepared from organisms grown in batch and continuous cultures (sulphur- or carbon-limited). The rate of the slowest step (tetrahydrodipicolinate acetylase) limited the rate of lysine synthesis to about 10 nmol min-l (mg protein)-', though none of the other enzymes was very much more active. Aspartokinase, aspartic P-semialdehyde dehydrogenase, dihydrodipicolinate synthase and N-acetyl-aminoketopimelate : glutamate aminotransferase were partly repressed by methion- ine; aspartokinase was also partly repressed by threonine. The aspartokinase repressed by methionine was inhibited by methionine plus lysine (though not by either amino acid singly), and the aspartokinase repressed by threonine was inhibited by threonine. None of the other repressible enzymes was inhibited by these amino acids. The activities of enzymes of glutamate and ornithine biosynthesis were very little altered by growth with limited or excess sulphate.

Activities and Regulation of the Enzymes of Lysine Biosynthesis in a Lysine-excreting Strain of Bacillus megaterium