Component of the ubiquinol-cytochrome c reductase complex (complex III or cytochrome b-c1 complex), which is a respiratory chain that generates an electrochemical potential coupled to ATP synthesis. c1 functions as an electron donor to cytochrome c. Binds 2 heme b groups non-covalently. Heme 1 (or bL or b562) is low-potential and absorbs at about 562 nm, and heme 2 (or bH or b566) is high-potential and absorbs at about 566 nm. Binds 1 heme c group covalently. Heme 3 is covalently bound to cytochrome c1. The main subunits of complex b-c1 are: cytochrome b, cytochrome c1 and the Rieske protein. Required for nitrogen fixation in root nodules on soybeans, but not for aerobic growth. It seems to be expressed under many growth conditions (aerobic, microaerobic and in nodule bacteroids). The protein is post-translationally processed into cytochrome b and c1. This occurs by processing between residues 434 and 435 without processing between cytochrome b and the N-terminal of the putative signal sequence domain. Belongs to the cytochrome b family.