The 18 kDa cytochrome c553 from Heliobacterium gestii: gene sequence and characterization of the mature protein

The 18 kDa cytochrome c553 is the dominant c-type cytochrome in cell membranes of Heliobacterium gestii. After solubilization, this cytochrome was purified in three steps as a complex with two other proteins of 32 and 42 kDa. The redox midpoint potential of the cytochrome c553 was determined to be +215 mV. The EPR spectra clearly show the presence of an ascorbate-reducible low-spin heme with gz = 3.048 and gy = 2.238. The gx = trough could not be detected. In addition, a Cu(II) signal with g = 2.058 was observed, indicating that one component of the cytochrome c553 complex contains a bound copper ion. The gene for the 18 kDa cytochrome c553, cyhA, consists of 429 bp coding for a protein of 142 amino acids. The association of the cytochrome with the cytoplasmic membrane is mediated by two fatty acid molecules, one palmitate and one stearate, that could be identified by mass spectrometry. Both fatty acids are most likely bound to the cysteine residue of the N-terminally processed protein via a glycerol moiety. The amino acid sequence deduced from the DNA sequence exhibits partial identity to the membrane-bound cytochrome c551 from Bacillus PS3 [Fujiwara, Y., Oka, M., Hamamoto, T., and Sone, N. (1993) Biochem. Biophys. Res. Commun. 1144, 213-219] and to the cytochrome c subunit (NorC) of the nitrous reductase from Pseudomonas stutzeri [Zumft, W. G., Braun, C., and Cuypers, H. (1994) Eur. J. Biochem. 219, 481-490].