Molecular evidence for the aerobic expression of nifJ, encoding pyruvate:ferredoxin oxidoreductase, in cyanobacteria.

Pyruvate:ferredoxin (flavodoxin) oxidoreductase (PFO, EC 1.2.7.1) catalyses the oxidative cleavage of pyruvate and coenzyme A to acetylcoenzyme A and CO2 with the simultaneous reduction of ferredoxin or flavodoxin. PFO occurs in anaerobes and in some aerobic archaea and bacteria. For cyanobacteria, activity measurements indicated the occurrence of PFO in heterocystous forms. The completely sequenced genomes of the unicellular Synechocystis sp. PCC 6803 and the heterocystous Anabaena sp. PCC 7120 and Nostoc punctiforme revealed the existence of one PFO (encoded by nifJ) in Synechocystis 6803 and N. punctiforme but two different PFOs, encoded by nifJ1 and nifJ2, in Anabaena. Sequence comparison now indicates that all cyanobacterial PFOs are more closely related to those of anaerobes than to those of aerobes. Reverse transcription-polymerase chain reaction (RT-PCR) experiments show that nifJ is transcribed in the presence of saturating iron concentrations in aerobically grown cells of the unicellular Synechococcus sp. PCC 6301 and Synechocystis 6803. Both nifJ genes are transcribed in aerobically grown Anabaena 7120. These findings are corroborated by luciferase reporter gene analysis of nifJ in Synechococcus sp. PCC 7942. The occurrence of PFO in these cyanobacteria is enigmatic.