Evidence for cytochrome oxidase subunit I and a cytochrome c– subunit II fused protein in the cytochrome ‘c1aa3’ of Thermus thermophilus

The terminal cytochrome c1aa3 of the respiratory chain of Thermus thermophilus has been isolated and purified to homogeneity by a novel procedure. The two subunit proteins (55 and 33 kDa) have been characterized chemically. Computer searches with partial amino acid sequences obtained from both subunits show that the larger subunit belongs to the cytochrome oxidase subunit I protein family while the smaller covalently heme-binding subunit is not a cytochrome c1 but appears to be a fused protein between cytochrome c and cytochrome oxidase subunit II. With respect to the 16-S rRNA-derived phylogeny of procaryotes, the results show that the genetic information for an O2-reacting cytochrome oxidase (EC 1.9.3.1) existed already in early eubacteria.