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

Abstract

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.

Single core polypeptide in the reaction center of the photosynthetic bacterium Heliobacillus mobilis: structural implications and relations to other photosystems.

“…mobilis possesses a true molecular mass of 68 kDa. This value is considerably larger than the electrophoretically determined 47-kDa value (10,12). Such large differences have also been observed for the RC subunits of purple bacteria, Chlorobiaceae, C. aurantiacus, and of PSI.…”

“…mobilis RC core polypeptide (Fig. 2) (3,4,10). In PSI, the RC core is formed by the two protein subunits PSI-A/B, of about 83 and 82 kDa, respectively (5,19,20).…”

“…Cleavage of the RC protein was performed chemically by cyanogen bromide and 2-(2'-nitrophenylsulfenyl)-3-methyl-3'-bromoindolenine and enzymatically by Clostripain. Peptide fragments were isolated and sequenced as described (10 …”

“…16S rRNA comparisons place the Heliobacteriaceae close to cyanobacteria and within the Gram-positive phylum for which no photosynthetic representatives had previously been identified (9). Structurally and functionally, the heliobacterial RC is similar to the RCs ofboth green sulfur bacteria and PSI (3,4,10).…”

Shared thematic elements in photochemical reaction centers.

“…This sequence occurs in the same relative position as in PSI and probably represents the binding site for iron-sulfur center Fx. A similar stretch of amino acids has been found in a proteolytic fragment of the H. mobilis RC (12). In both C. limicola and H. mobilis it is striking that the highly conserved aspartic acid residues that flank three of the cysteines in PSI-A and PSI-B are absent; the significance of this omission is not known.…”