Summary: The role of the blue copper protein azurin and cytochrome C as the possible electron donors to nitrite reductase in the dissimilatory nitrate reduction pathway in have been investigated. It was shown by an approach with mutant strains of deficient in one or both of these electron-transfer proteins that cytochrome C, but not azurin, is functional in this pathway. Expression studies demonstrated the presence of azurin in both aerobic and anaerobic cultures. A sharp increase in azurin expression was observed when cultures were shifted from exponential to stationary phase. The stationary-phase sigma factor, σ, was shown to be responsible for this induction. In addition, one of the two promoters transcribing the gene was regulated by the anaerobic transcriptional regulator ANR. An azurin-deficient mutant was more sensitive to hydrogen peroxide and paraquat than the wild-type . These results suggest a physiological role of azurin in stress situations like those encountered in the transition to the stationary phase.


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