the activity and synthesis of the Calvin cycle enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (RuBisCO) are regulated by inactivation and endproduct repression, respectively. Phosphoenolpyruvate (PEP) has been suggested to function as a signal molecule for the latter control system. During growth of the organism in carbon-source-limited continuous cultures with various ratios of acetate and formate in the feed, the RuBisCO levels varied considerably, but no correlation was observed with the intracellular concentrations of PEP. To study whether the repression exerted by acetate utilization was dependent on the synthesis of glycolytic intermediates from this compound, an acetate-negative mutant defective in isocitrate lyase was isolated and characterized. Clear evidence was obtained that in this mutant acetate is as effective in repressing RuBisCO synthesis as in the wild-type. It therefore appears more likely that acetyl-CoA or a closely related metabolite functions as a signal molecule in the regulation of RuBisCO synthesis.


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