Structural Superimposition

Abstact

Photosynthetic reaction center-light harvesting 1 (RC-LH1) core supercomplexes are essential for energy capture and electron transport in purple bacteria. Rhodospirillum rubrum, a model organism for bacterial photosynthesis, features an RC-LH1 architecture with a closed LH1 ring and lacks the peripheral LH2 antenna in the photosynthetic membranes. How this unique RC-LH1 supercomplex performs energy transfer and quinone transport remains unclear. Here, we characterized both the structural and functional properties of Rsp. rubrum RC-LH1 supercomplex using cryo-electron microscopy (cryo-EM), transient absorption (TA) spectroscopy, and cytochrome c(2) oxidation assays. Cryo-EM of the RC-LH1 monomeric structure revealed a closed LH1 ring of 16 alphabeta-polypeptides encircling the RC, with weaker RC-LH1 interactions than other RC-LH1 structures reported. TA spectra and cytochrome c(2) oxidation assays showed that Rsp. rubrum RC-LH1 monomer with a closed LH1 ring exhibits slower and more distributed excitation energy transfer (EET) kinetics from LH1 to RC and slower electron transport rates than Rba. sphaeroides RC-LH1 monomer with a large opening in the LH1 ring. Our findings provide insight into the unique architecture and spectroscopic properties of Rsp. rubrum RC-LH1 supercomplex. This study enhances our understanding of bacterial photosynthetic mechanisms and lays the foundation for bioengineering applications in artificial photosynthetic systems.