Protein sequences and redox titrations indicate that the electron acceptors in reaction centers from heliobacteria are similar to Photosystem I

• Published in: Photosynthesis research 1992-04, Vol.32 (1), p.11-22
• Main Authors: Trost, J. T., Brune, D. C., Blankenship, R. E.
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; Bacteria - genetics; Bacteria - metabolism; Bacterial Physiological Phenomena; Bacterial Proteins - analysis; Bacterial Proteins - chemistry; Bacterial Proteins - genetics; Biological and medical sciences; Biological Evolution; Cell structures and functions; Chloroplast, photosynthetic membrane and photosynthesis; Electrophoresis, Polyacrylamide Gel; Exobiology; Fundamental and applied biological sciences. Psychology; Iron; Molecular and cellular biology; Molecular Sequence Data; Oxidation-Reduction; Photosynthesis - genetics; Photosynthesis - physiology; Photosynthetic Reaction Center Complex Proteins - chemistry; Photosynthetic Reaction Center Complex Proteins - metabolism; Photosystem I Protein Complex; Sequence Homology, Amino Acid; Space life sciences; Sulfur; Titrimetry
• ISSN: 0166-8595; 1573-5079
• DOI: 10.1007/BF00028794

Photosynthetic reaction centers isolated from Heliobacillus mobilis exhibit a single major protein on SDS-PAGE of 47 000 Mr. Attempts to sequence the reaction center polypeptide indicated that the N-terminus is blocked. After enzymatic and chemical cleavage, four peptide fragments were sequenced from the Heliobacillus mobilis apoprotein. Only one of these sequences showed significant specific similarity to any of the protein and deduced protein sequences in the GenBank data base. This fragment is identical with 56% of the residues, including both cysteines, found in highly conserved region that is proposed to bind iron-sulfur center Fx in the Photosystem I reaction center peptide that is the psaB gene product. The similarity to the psaA gene product in this region is 48%. Redox titrations of laser-flash-induced photobleaching with millisecond decay kinetics on isolated reaction centers from Heliobacterium gestii indicate a midpoint potential of -414 mV with n = 2 titration behavior. In membranes, the behavior is intermediate between n = 1 and n = 2, and the apparent midpoint potential is -444 mV. This is compared to the behavior in Photosystem I, where the intermediate electron acceptor A1, thought to be a phylloquinone molecule, has been proposed to undergo a double reduction at low redox potentials in the presence of viologen redox mediators. These results strongly suggest that the acceptor side electron transfer system in reaction centers from heliobacteria is indeed analogous to that found in Photosystem I. The sequence similarities indicate that the divergence of the heliobacteria from the Photosystem I line occurred before the gene duplication and subsequent divergence that lead to the heterodimeric protein core of the Photosystem I reaction center.