Abstract
The gcr of Bacillus stearothermophilus K1041 encoding three subunits of the quinol-cytochrome c oxidoreductase (cytochrome reductase, b6c1 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 c1 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 b6 of subunit 25,425 Da with 224 residues (Sone, N., Sawa, G., 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 c1 was not similar to either type; the thermophilic Bacillus cytochrome c1 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 b6c1 complex is unique.
Highlights
The qcr of Bacillus stearothermophilus K1041 encoding three subunits of the quinol-cytochrome c oxidoreductase was cloned and sequenced
The presented data demonstrate that the structural genes for three subunits of thermophilic Bacillus cytochrome reductase constitute an operon structure like that of the proteobacterial fbc operon
The previously reported qcrB (petB) gene for cytochrome b6 of cyanobacteria is followed by petD for subunit IV, which has sequence homology with the C-terminal half of cytochrome b of proteobacteria [14, 15]
Summary
Cytochrome b6, and a Novel-type Cytochrome c1 of Quinolcytochrome c Reductase*. 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 b6c1 complex is unique. We cloned the cytochrome b locus from transformable B. stearothermophilus K1041 (Bst) and found that the gene for cytochrome b6 encodes a 224-aa protein with four hydrophobic helices possessing two pairs of His as ligands for the two protoheme, while the gene for subunit IV corresponds to the C-terminal half of cytochrome b [18]. The sequence of Bst cytochrome c1 is quite different from that of purple bacterial cytochrome c1; the Bst c1 is composed of a membrane domain homologous to subunit IV and a hydrophilic part homologous to Bacillus small cytochrome c (Refs. 24 and 25; see Ref. 26 for review)
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