| Abstrakt: |
The gene encoding Desulfovibrio desulfuricansNorway cytochrome c3(Mr26,000), a dimeric octaheme cytochrome belonging to the polyheme cytochrome c3superfamily, has been cloned and successfully expressed in another sulfate reducing bacteria, D. desulfuricansG201. The gene, named cycD, is monocistronic and encodes a cytochrome precursor of 135 amino acids with an extension at the NH2terminus of 24 amino acids. This extension acts as a signal sequence which allows export across the cytoplasmic membrane into the periplasmic space. Tyrosine 73, which is in a close contact with the histidine sixth axial ligand to the heme 4 iron atom, has been replaced by a glutamate residue using site-directed mutagenesis. The cytochrome mutant when expressed in D. desulfuricansG201, is correctly folded and matured. A global increase of the oxidoreduction potentials of about 50 mV is measured for the Y73E cytochrome. The mutation also has a strong influence on the interaction of the cytochrome with its redox partner, the hydrogenase. This suggests, like the tetraheme cytochrome c3(Mr13,000), heme 4 is the interactive heme in the cytochrome-hydrogenase complex and that alteration of the heme 4 environment can greatly affect the electron transfer reaction with its redox partner. |
