Component of the green S-bacteria bc complex, which consists of the Rieske protein and cytochrome b subunit but appears to lack a cytochrome c1-equivalent. This complex has a comparatively low redox potential (By similarity). 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. Belongs to the cytochrome b family.