Expression and content of terminal oxidases in Azotobacter vinelandii grown with excess NH4+ are modulated by O-2 supply

The influence of the rate of O-2 supply to batch cultures on the contents of cytochromes bd and 'o' in NH4+-grown Azotobacter vinelandii has been investigated, Difference spectra at room temperature (reduced + CO minus reduced) were recorded for whole cells of a wild-type strain and mutants which either lacked or over-produced the cytochrome bd-type terminal oxidase encoded by cydAB, A Tn5-B20 insertion in cydB in the former mutant also provided a means of monitoring cydAB gene expression from measurements of beta-galactosidase activity. The content of cytochrome d in the wild-type, and the expression of cydAB-lacZ, in the mutant, increased as the O-2 supply was raised, suggesting that O-2 regulates cydAB expression even in the absence of diazotrophy. In a strain carrying a mutation in cydR, a regulatory gene upstream of cydAB, and which over-produces cytochrome bd. the responses to O-2 supply during growth at different O-2 supply rates were reversed. Changes in the content of a haemoprotein detectable in low temperature photodissociation spectra, and attributed to cytochrome b(595) - the high-spin cytochrome b component of the cytochrome bd complex - followed the changes in cytochrome d levels. CO difference spectra of both the wild-type strain and the cytochrome bd-deficient mutant revealed a haemoprotein with spectral characteristics similar to cytochrome o, the levels of which increased as the O-2 supply was raised. These results are discussed with reference to previous reports of cytochrome changes in cells grown under N-2-fixing conditions.