Cyclic AMP and citric acid accumulation by Aspergillus niger

Aspergillus niger accumulated citric acid in the medium under certain conditions. Cyclic AMP concentrations of the order of 10^?6M and higher caused an increase in the rate of citrate synthesis. Adenosine, ATP, and cyclic GMP at 10^?3M also stimulated, but were ineffective at 10^?4M. 5′-AMP had no effect while 5′-GMP and guanosine inhibited slightly. ADP showed a 42% inhibition. Theophylline enhanced the cyclic AMP effect. It is proposed that citric acid accumulation by Aspergillus niger may result from abnormal cyclic AMP metabolism.