CLONING AND SEQUENCE-ANALYSIS OF THE STRUCTURAL GENE FOR THE BC(1)-TYPE RIESKE IRON-SULFUR PROTEIN FROM THERMUS-THERMOPHILUS HB8

The structural gene encoding the Rieske iron-sulfur protein from Therm us thermophilus HB8 has been cloned and sequenced. The gene encodes a protein of 209 amino acids that begins with a hydrophilic N-terminus f ollowed by a stretch of 21 hydrophobic amino acids that could serve as a transmembrane helix. The remainder of the protein has a hydrophobic ity pattern typical of a water-soluble protein. A phylogenetic analysi s of 26 Rieske proteins that are part of be I or hbf complexes shows t hat they fall into three major groups: eubacterial and mitochondrial, cyanobacterial and plastid, and five highly divergent outliers, includ ing that of Thermus. Although the overall homology with other Rieske p roteins is very low, the C-terminal half of the Thermus protein contai ns the signature sequence CTHLGC-(13X)-CPCH that most likely provides the ligands of the [2Fe-2S] cluster. It is proposed that this region o f the protein represents a small domain that folds independently and t hat the encoding DNA sequence may have been transferred during evoluti on to several unrelated genes to provide the cluster attachment site t o proteins of different origin. The role of individual residues in thi s domain of the Thermus protein is discussed vis-a-vis the three-dimen sional structure of the bovine protein.