1. 5-Aminolaevulinate synthetase was detected in extracts of the non-photosynthetic bacterium Micrococcus denitrificans. 2. Activity is high in cells grown anaerobically in a defined nitrate medium, but is low in cells grown in an iron-deficient medium, and in cells grown aerobically. 3. Aminolaevulinate synthetase was purified extensively; it has a molecular weight of about 68000; apparent Km values for glycine, succinyl-CoA and pyridoxal phosphate are 12mm, 10μm and 11μm respectively; 2μm-haemin and 14μm-protoporphyrin inhibit by 50%. 4. The low activity of aminolaevulinate synthetase in iron-deficient cells increases on adding iron salts to cells only under conditions where protein synthesis can occur. 5. In defined nitrate medium with a high Ca2+ concentration anaerobic growth yield is higher, but aminolaevulinate synthetase activity is lower than in cells grown in the medium with a low Ca2+ concentration. In medium made from AnalaR constituents, growth yield is low and aminolaevulinate synthetase activity is high even in the presence of high concentrations of Ca2+; on adding Cu2+ (0.1μm) to the medium growth yield and aminolaevulinate synthetase activity become the same as in non-AnalaR medium. 6. Cells incubated under conditions where protein synthesis does not occur but where electron transport does, lose their aminolaevulinate synthetase activity rapidly. 7. The activities of aminolaevulinate dehydratase and succinic thiokinase do not change under any of the conditions of growth examined. 8. The possible mechanisms controlling aminolaevulinate synthetase activity and the role of this enzyme in controlling the synthesis of haem in this organism are discussed.

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