Heliobacterial photosynthesis

Area of Science:

• Photosynthesis research
• Biochemistry of electron transfer
• Structure and function of reaction centers

Background:

• Heliobacteria possess Type I reaction centers (RCs) but lack extensive antenna systems, making them ideal for studying simpler RCs.
• Previous knowledge on heliobacterial RCs (HbRCs) was limited, especially regarding secondary and tertiary electron acceptors.
• Significant gaps existed in understanding HbRC polypeptide composition, electron acceptor chain, and electron transfer kinetics.

Purpose of the Study:

• To elucidate the nature of secondary and tertiary electron acceptors in the heliobacterial reaction center (HbRC).
• To characterize the polypeptide composition and electron transfer kinetics of HbRCs.
• To identify and clone the gene encoding the protein containing terminal iron-sulfur clusters FA and FB.

Main Methods:

• Spectroscopic analysis (EPR and Mössbauer) to identify the FX cluster.
• Chemical assays to determine the ratio of Bchl g to P798.
• N-terminal amino acid sequencing to identify and clone the gene for the FA/FB-containing polypeptide.
• Reconstitution of expressed protein into isolated HbRC cores.

Main Results:

• The low molecular mass polypeptide containing iron-sulfur clusters FA and FB was identified as PshB.
• Removal of PshB altered flash-induced kinetics, indicating its role in P798+ [FA/FB]- recombination (75 ms).
• The FX cluster was identified as a [4Fe-4S]1+,2+ cluster with S=3/2, and all iron in HbRC cores resides in FX.
• The gene 'pshB' was identified and cloned; expressed PshB restored 75 ms recombination kinetics and EPR signals.

Conclusions:

• The PshB protein and FX cluster are crucial components of the heliobacterial reaction center's electron acceptor chain.
• Detailed characterization of HbRC structure and function has significantly advanced.
• This work provides a foundation for understanding prototypical homodimeric Type I reaction centers.