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


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