Abstract
A photosynthetic reaction center complex has been isolated from the green sulfur bacterium Chlorobium vibrioforme. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis reveals polypeptides with apparent molecular masses of 80, 40, 18, 15, 9, and 6 kDa. Only the 18-kDa polypeptide is stained with 3,3',5,5'-tetramethylbenzidine, a heme-specific reagent. Oxidized minus reduced difference spectra show the presence of approximately one heme/P840 and the presence of a cytochrome c551. Flash photolysis of P840 was followed by rereduction of P840+ and oxidation of cytochrome c551, both with a biphasic kinetic with t1/2 values of 7 and 50 microseconds. Using oligonucleotide probes derived from an N-terminal amino acid sequence of the 18-kDa polypeptide, a genomic clone was isolated. The sequence of the gene, which we designate cycA, predicts a single heme binding site (Cys-Asn-Lys-Cys-His). The 621-base pair open reading frame encodes an apoprotein of 22,858 Da with three predicted membrane-spanning alpha-helices. No extensive sequence similarity is found to other cytochromes. Northern blotting indicates that the cycA gene is transcribed as a monocistronic mRNA. Southern blotting shows the presence of only one cycA gene in the C. vibrioforme and Chlorobium tepidum genomes. The unique membrane-bound monoheme cytochrome c551 of C. vibriforme is assigned to a new class of c-type cytochromes. The implications for the current view of evolution of photosynthetic reaction center complexes are discussed.
Highlights
A Membrane-bound Monoheme Cytochromec551 of a Novel Type Isthe Immediate Electron Donor to P840 of the Chlorobium vibrioforme Photosynthetic Reaction Center Complex*
On the donor side of the reaction center complex of green sulfur bacteria, c-type cytochromes are known to function as electron donors to photooxidized P840 [10, 11, 16,17,18,19,20]
In this paper we report that the reaction center complex from Chlorobiumuibrioforme containscytochrome c5$, in a ratio of one heme/P840
Summary
A Membrane-bound Monoheme Cytochromec551 of a Novel Type Isthe Immediate Electron Donor to P840 of the Chlorobium vibrioforme Photosynthetic Reaction Center Complex*. On the donor side of the reaction center complex of green sulfur bacteria, c-type cytochromes are known to function as electron donors to photooxidized P840 [10, 11, 16,17,18,19,20]. Quantitation of cytochrome c in membranes from green sulfur bacteria generally show a content of four to six hemes/ P840 decreasing to one to two in purified reaction center preparations [10, 11,17,18,19,20].
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