Bacillus stearothermophilus qcr operon encoding Rieske FeS protein, cytochrome b(6), and a novel-type cytochrome c(1) of quinol-cytochrome c reductase

The qcr of Bacillus stearothermophilus K1041 encoding three subunits of the quinol-cytochrome c oxidoreductase (cytochrome reductase, b(6)c(1) complex) was cloned and sequenced. The gene (qcrA) for a Rieske FeS protein of 19,144 Da with 169 amino acid residues, and the gene (qcrC) for cytochrome c(1) of 27,342 Da with 250 amino acid residues were found at adjacent upstream and downstream sides of the previously reported qcrB (petB) for cytochrome b(6) of subunit 25,425 Da with 224 residues (Sone, N., Sawa, C., Sone, T., and Noguchi, S. (1995) J. Biol. Chem. 270, 10612-10617). The three structural genes for thermophilic Bacillus cytochrome reductase form a transcriptional unit. In the deduced amino acid sequence for the FeS protein, the domain including four cysteines and two histidines binding the 2Fe-2S cluster was conserved. Its N-terminal part more closely resembled the cyanobacteria-plastid type than the proteobacteria-mitochondria type when their sequences were compared. The amino acid sequence of cytochrome c(1) was not similar to either type; the thermophilic Bacillus cytochrome c(1) is composed of an N-terminal part corresponding to subunit IV with three membrane spanning segments, and a C-terminal part of cytochrome c reminiscent of cytochrome c-551 of thermophilic Bacillus. The subunit IV in the enzyme of cyanobacteria and plastids is the counterpart of C-terminal part of cytochrome b of proteobacteria and mitochondria. These characteristics indicate that Bacillus cytochrome b(6)c(1) complex is unique.