THE CYTOCHROME-BC COMPLEX (MENAQUINONE-CYTOCHROME-C REDUCTASE) IN BACILLUS-SUBTILIS HAS A NONTRADITIONAL SUBUNIT ORGANIZATION

We have identified an operon in Bacillus subtilis, designated qcr, that is thought to encode a quinone: cytochrome c reductase, Northern (RNA blot) analysis suggests a tricistronic operon. The operon is located at about 200 degrees on the B. subtilis map, Disruption of the operon leads to loss of a 22-kDa cytochrome c from membrane preparations. The structure of the putative protein products of the qcr operon suggests a protein complex that is closely related to but distinct from known cytochrome bc(1) and b(6)f complexes, which catalyze electron transfer from a quinol to a c-type cytochrome or to plastocyanin. QcrA is similar to Rieske-type iron-sulfur proteins; QcrB is similar in size and sequence to b type cytochromes from b(6)f complexes; and QcrC has a novel structure that resembles a fusion of a subunit IV (found in b(6)f complexes) to a cytochrome c, Transcription of the operon is induced at the end of exponential growth from a sigma(A)-like promoter, This transition state induction appears to be dependent on the downregulation of abrB expression, which is mediated by SpoOA activation. As bacteria move from the transition state into sporulation, transcription of the operon is reduced in a sigma(F)-dependent manner.