Heliobacterial photosynthesis

Heliobacteria contain Type I reaction centers (RCs) and a homodimeric core, but unlike green sulfur bacteria, they do not contain an extended antenna system. Given their simplicity, the heliobacterial RC (HbRC) should be ideal for the study of a prototypical homodimeric RC. However, there exist enormous gaps in our knowledge, particularly with regard to the nature of the secondary and tertiary electron acceptors. To paraphrase S. Neerken and J. Amesz (2001 Biochim Biophys Acta 1507:278–290): with the sole exception of primary charge separation, little progress has been made in recent years on the HbRC, either with respect to the polypeptide composition, or the nature of the electron acceptor chain, or the kinetics of forward and backward electron transfer. This situation, however, has changed. First, the low molecular mass polypeptide that contains the terminal F_A and F_B iron-sulfur clusters has been identified. The change in the lifetime of the flash-induced kinetics from 75 ms to 15 ms on its removal shows that the former arises from the P798⁺ F_A/F_B]^? recombination, and the latter from P798⁺ F_X ^? recombination. Second, F_X has been identified in HbRC cores by EPR and Mössbauer spectroscopy, and shown to be a 4Fe–4S]^1+,2+ cluster with a ground spin state of S = 3/2. Since all of the iron in HbRC cores is in the F_X cluster, a ratio of ～22 Bchl g/P798 could be calculated from chemical assays of non-heme iron and Bchl g. Third, the N-terminal amino acid sequence of the F_A/F_B-containing polypeptide led to the identification and cloning of its gene. The expressed protein can be rebound to isolated HbRC cores, thereby regaining both the 75 ms kinetic phase resulting from P798⁺ F_A/F_B]^? recombination and the light-induced EPR resonances of F_A ^? and F_B ^?. The gene was named ‘pshB’ and the protein ‘PshB’ in keeping with the accepted nomenclature for Type I RCs. This article reviews the current state of knowledge on the structure and function of the HbRC.