Auracyanin A from the thermophilic green gliding photosynthetic bacterium Chloroflexus aurantiacus represents an unusual class of small blue copper proteins

The amino acid sequence of the small copper protein auracyanin A isolated f rom the thermophilic photosynthetic green bacterium Chloroflexus aurantiacu s has been determined to be a polypeptide of 139 residues. His58, Cys123, H is128, and Met132 are spaced in a way to be expected if they are the evolut ionary conserved metal ligands as in the known small copper proteins plasto cyanin and azurin. Secondary structure prediction also indicates that aurac yanin has a general beta-barrel structure similar to that of azurin from Ps eudomonas aeruginosa and plastocyanin from poplar leaves. However, auracyan in appears to have sequence characteristics of both small copper protein se quence classes. The overall similarity with a consensus sequence of azurin is roughly the same as that with a consensus sequence of plastocyanin, name ly 30.5%. We suggest that auracyanin A, together with the B forms, is the f irst example of a new class of small copper proteins that may be descendant s of an ancestral sequence to both the azurin proteins occurring in prokary otic nonphotosynthetic bacteria and the plastocyanin proteins occurring in both prokaryotic cyanobacteria and eukaryotic algae and plants. The N-termi nal sequence region 1-18 of auracyanin is remarkably rich in glycine and hy droxy amino acids, and required mass spectrometric analysis to be determine d. The nature of the blocking group X is not yet known, although its mass h as been determined to be 220 Da. The auracyanins are the first small blue c opper proteins found and studied in anoxygenic photosynthetic bacteria and are Likely to mediate electron transfer between the cytochrome bc(1) comple x and the photosynthetic reaction center.