IN-VIVO STUDIES DISPROVE AN OBLIGATORY ROLE OF AZURIN IN DENITRIFICATION IN PSEUDOMONAS-AERUGINOSA AND SHOW THAT AZU EXPRESSION IS UNDER CONTROL OF RPOS AND ANR

The role of the blue copper protein azurin and cytochrome c(551) as th e possible electron donors to nitrite reductase in the dissimilatory n itrate reduction pathway in Pseudomonas aeruginosa have been investiga ted. It was shown by an in vivo approach with mutant strains of P. aer uginosa deficient in one or both of these electron-transfer proteins t hat cytochrome c(551), 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 obs erved when cultures were shifted from exponential to stationary phase. The stationary-phase sigma factor, sigma(s), was shown to be responsi ble for this induction. In addition, one of the two promoters transcri bing the azu gene was regulated by the anaerobic transcriptional regul ator ANR. An azurin-deficient mutant was more sensitive to hydrogen pe roxide and paraquat than the wild-type P. aeruginosa. These results su ggest a physiological role of azurin in stress situations like those e ncountered in the transition to the stationary phase.