Cytochrome bd biosynthesis in Bacillus subtilis: Characterization of the cydABCD operon

Under aerobic conditions Bacllus subtilis utilizes a branched electron tran sport chain comprising various cytochromes and terminal oxidases. At presen t there is evidence for three types of terminal oxidases in B. subtilis: a caa(3)-, an aa(3)-, and a bd-type oxidase. We report here the cloning of th e structural genes (cydA and cydB) encoding the cytochrome bd complex. Down stream of the structural genes, cydC and cydD are located. These genes enco de proteins showing similarity to bacterial ATP-binding cassette (ABC)-type transporters. Analysis of isolated cell membranes showed that inactivation of cydA or deletion of cydABCD resulted in the loss of spectral features a ssociated with cytochrome bd. Gene disruption experiments and complementati on analysis showed that the cydC and cydD gene products are required for th e expression of a functional cytochrome bd complex. Disruption of the cyd g enes had no apparent effect on the growth of cells in broth or defined medi a. The expression of the cydABCD operon was investigated by Northern blot a nalysis and by transcriptional and translational cyd-lacZ fusions. Northern blot analysis confirmed that cydABCD is transcribed as a polycistronic mes sage. The operon was found to be expressed maximally under conditions of lo w oxygen tension.