Abstract: Auracyanins are blue copper proteins produced by the anoxygenic photosynthetic bacterium Chloroflexus aurantiacus. They function as electron carriers in this organism, which lacks soluble iron proteins as electron-transfer agents. The two forms that have been discovered, A and B, have very similar globular domains but have distinct N-terminal tails that tether the proteins to the cytoplasmic membrane, suggesting differences in locations and functions. The genes for the globular domains of auracyanins have been cloned in a bacterial expression system enabling purification of large quantities of proteins for spectroscopic and structural studies. The X-ray crystal structure of auracyanin A, reported here, showed a polypeptide fold similar to the one previously published for auracyanin B. The structure analysis, as well as X-ray absorption spectroscopy, showed that the copper site dimensions in the two auracyanins have very small structural differences. However, the electron paramagnetic resonance spectra are very different, indicating a difference in symmetry without a significant difference in the coordinates of the copper site. Measurements done for C. aurantiacus cells grown at different conditions showed that auracyanin A is expressed during photosynthetic growth, while auracyanin B is expressed in both photosynthetic and aerobic respiratory growth. A newly discovered multisubunit membrane-bound electron transfer complex has also been purified from C. aurantiacus. It has subunits homologous to the three-subunit molybdopterin oxidoreductases and four additional subunits, two of which are c-type cytochromes. Putative operons for similar complexes were identified in other bacteria, most of which do not contain a cytochrome bc or bf electron transfer complex, suggesting functional replacement.