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, tha t is thought to encode a quinone: cytochrome c reductase, Northern (RN A 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. Th e structure of the putative protein products of the qcr operon suggest s a protein complex that is closely related to but distinct from known cytochrome bc(1) and b(6)f complexes, which catalyze electron transfe r from a quinol to a c-type cytochrome or to plastocyanin. QcrA is sim ilar 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 no vel 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 a t the end of exponential growth from a sigma(A)-like promoter, This tr ansition state induction appears to be dependent on the downregulation of abrB expression, which is mediated by SpoOA activation. As bacteri a move from the transition state into sporulation, transcription of th e operon is reduced in a sigma(F)-dependent manner.