The recent discovery that phosphoenolpyruvate carboxylase (PEPCx) is dispensable for growth and lysine production in implies that this organism possesses (an) alternative anaplerotic enzyme(s). In permeabilized cells of we detected pyruvate carboxylase (PCx) activity. This activity was effectively inhibited by low concentrations of ADP, AMP and acetyl-CoA. PCx activity was highest [45.5 nmol min (mg dry wt)] in cells grown on lactate or pyruvate, and was about two- to threefold lower when the cells were grown on glucose or acetate, suggesting that formation of PCx is regulated by the carbon source in the growth medium. In cells grown at low concentrations of biotin (< 5 μg I), PCx activity was drastically reduced, indicating that the enzyme is a biotin protein. Growth experiments with the wild-type and a defined PEPCx-negative mutant of on glucose showed that the mutant has a significantly higher demand for biotin than the wild-type, whereas both strains have the same high biotin requirement for growth on lactate and the same low biotin requirement for growth on acetate. These results indicate that (i) PCx is an essential anaplerotic enzyme for growth on glucose in the absence of PEPCx, (ii) PCx is an essential anaplerotic enzyme for growth on lactate even in the presence of PEPCx, and (iii) PCx has no anaplerotic significance for growth on acetate as the carbon source. In support of these conclusions, screening for clones unable to grow on a minimal medium containing lactate, but able to grow on a medium containing glucose or acetate, led to the isolation of PCx-defective mutants of


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