SUMMARY: The enzymes λ-aminolaevulic acid synthetase and λ-aminolaevulic acid dehydrase are concerned in the early stages of tetrapyrrole formation; factors controlling their synthesis have been studied in cultures of growing exponentially. In organisms growing in the dark under high degrees of aeration the differential rate of synthesis (increase in enzyme/increase in culture density) of both enzymes is about one-third of that in cultures growing anaerobically in the light. Organisms growing in the dark under low degrees of aeration form the enzymes at rates comparable to those in photosynthetic cultures. Under anaerobic conditions the differential rate of synthesis of both enzymes is decreased by increasing the light intensity. Enzyme synthesis in the light is repressed by oxygen, the effect being overcome on restoration of anaerobic conditions. Formation of bacterio-chlorophyll under these various conditions is affected in the same way as enzyme synthesis. Addition of λ-aminolaevulic acid or haemin to growing cultures stopped the synthesis of the synthetase and the dehydrase; magnesium protoporphyrin had no such effect. The activity of the synthetase, but not of the dehydrase, was decreased in organisms growing with suboptimal concentrations of biotin. A component of the electron transport chain as well as the intracellular concentration of biosynthetic intermediates may contribute, by mechanisms as yet unknown, to the regulation of synthesis of enzymes involved in bacteriochlorophyll formation.


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