Summary: The effect of carbon growth substrate on the level of ribulose-1,5-bisphosphate (RuBP) carboxylase in crude cell-free extracts of (RM5) was investigated. The highest specific activity followed growth with sodium hydrogen malate as carbon source. Cells grown autotrophically or on the reduced carbon substrates, butyrate and ethanol, gave extracts of lower activity. RuBP carboxylase was purified from (RM5) grown on a mixture of pyruvate and malate as carbon source. The enzyme was homogeneous as judged by electrophoresis on polyacrylamide gels of several acrylamide concentrations and had an apparent molecular weight of 430000 as measured by gel filtration. Sodium dodecyl sulphate polyacrylamide gel electrophoresis suggested the presence of three subunit types of molecular weights 56200, 53300 and 15700. Mg was required for maximum enzyme activity although this could be completely replaced by Ni and, to a lesser extent, by Mn or Co. As with other high molecular weight RuBP carboxylases, the enzyme was sensitive to inhibition by 6-phosphogluconate.


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