Deletion of the 6-kDa subunit affects the activity and yield of the bc(1) complex from Rhodovulum sulfidophilum

The cytochrome bc(1) complex from Rhodovulum sulfidophilum purifies as a fo ur-subunit complex: the cytochrome b, cytochrome c(1) and Rieske iron-sulph ur proteins, which are encoded together in the fbc operon, as well as a 6-k Da protein. The gene encoding the 6-kDa protein, named fbcS, has been ident ified. It is located within the sox operon, which encodes the subunits of s arcosine oxidase. The encoded 6-kDa protein is very hydrophobic and is pred icted to form a single transmembrane helix. It shows no sequence homology t o any known protein. The gene has been knocked-out of the genome and a thre e-subunit complex can be purified. This deletion leads to a large reduction in the yield of the isolated complex and in its activity compared to wild- type. The high quinone content found in the wild-type complex is, however, maintained after removal of the 6-kDa protein. Surprisingly, a fourth subun it of approximately 6 kDa is again found to copurify with the Rhv. sulfidop hilum bc(1) complex when only the fbc operon is expressed heterologously in a near-relative, Rhodobacter capsulatus, which lacks this small subunit in its own bc(1) complex.