The requirement of ArcA and Fnr for peak expression of the cyd operon in Escherichia coli under microaerobic conditions.

Transcriptional regulation of cyd, encoding the cytochrome d complex for O2 scavenging, was studied in Escherichia coli by monitoring phi(cyd-lac) expression under atmospheres containing 0-21% O2. Peak expression of this operon occurred under microaerobic conditions. Mutations in arcA (a trans-acting regulatory gene involved in aerobic respiration) greatly lowered phi(cyd-lac) expression under all conditions. Mutations in fnr (a trans-acting regulatory gene involved in anaerobic respiration) shifted the pattern of phi(cyd-lac) expression, lowering the microaerobic and aerobic expression. In an arcA-fnr double mutant, phi(cyd-lac) expression became insignificant, irrespective of the availability of O2. It thus appears that the expression of the phi(cyd-lac) operon is under dual control by the two pleiotropic activators, ArcA and Fnr, which interact to give the peak microaerobic expression.