A membrane-bound monoheme cytochrome c551 of a novel type is the immediate electron donor to P840 of the Chlorobium vibrioforme photosynthetic reaction center complex.
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.
Okkels
J S
JS
Department of Plant Biology, Royal Veterinary and Agricultural University, Frederiksberg C, Copenhagen, Denmark.
Kjaer
B
B
Hansson
O
O
Svendsen
I
I
Møller
B L
BL
Scheller
H V
HV
D000595:Amino Acid Sequence; D001426:Bacterial Proteins; D001483:Base Sequence; D015152:Blotting, Northern; D015139:Blotting, Southern; D003574:Cytochrome c Group / Q000235:genetics* / Q000378:metabolism*; D004269:DNA, Bacterial / Q000235:genetics / Q000302:isolation & purification; D004579:Electron Transport; D004591:Electrophoresis, Polyacrylamide Gel; D005798:Genes, Bacterial; D006418:Heme / Q000378:metabolism; D007700:Kinetics; D008027:Light; D008969:Molecular Sequence Data; D008970:Molecular Weight; D045322:Photosynthetic Reaction Center Complex Proteins / Q000302:isolation & purification / Q000378:metabolism*; D011487:Protein Conformation; D012329:RNA, Bacterial / Q000235:genetics / Q000302:isolation & purification; D015183:Restriction Mapping; D013053:Spectrophotometry
D001426:Bacterial Proteins; D003574:Cytochrome c Group; D004269:DNA, Bacterial; D045322:Photosynthetic Reaction Center Complex Proteins; D012329:RNA, Bacterial; D006418:Heme; C017167:cytochrome C(551)