On the role of cyclic AMP and the Fnr protein in Escherichia coli growing anaerobically.

The role of adenosine 3',5'-monophosphate (cAMP) and of the Fnr protein, a transcriptional regulator of anaerobic electron transport, in the expression of anaerobic respiration of Escherichia coli was investigated. Under conditions of fermentation or anaerobic respiration intracellular cAMP was formed in concentrations up to 4.6 nmol/g protein. From the enzymes of the anaerobic electron transfer chain from glycerol-3-P to fumarate only the expression of glycerol-3-P dehydrogenase (Freedberg WB, Lin ECC (1973) J Bacteriol 115:816-823), but not that of fumarate reductase required cAMP. Isolated Fnr protein, which has been suggested to be an additional site of action of cAMP under anaerobic conditions did not bind cAMP. It is concluded that cAMP in anaerobic growth like in aerobic growth acts as the effector of CRP and that catabolite repression plays an important regulatory role in anaerobic catabolism. The Fnr protein was present in constant amounts (0.06 mg/g cellular protein) and in constant molar mass (Mr 30,000) in aerobically and in anaerobically grown bacteria. This result excluded regulation of the activity of the Fnr protein by a change of concentration or by processing of the protein.