Expression and Content of Terminal Oxidases in Azotobacter Vinelandii Grown with Excess NH4+ are Modulated by O2 Supply

The influence of the rate of O 2 supply to batch cultures on the contents of cytochromes bd and ‘o’ in NH 4 + -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 β-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.