Assembly, selectivity, and compatibility of bacterial photosynthetic complexes from divergent species detected in a...

Photosynthetic complexes comprising light-harvesting (LH) and reaction center (RC) components are essential for biological energy conversion in photosynthesis. Assembly of these multi-protein structures is a topic of great interest, and assembly mechanisms appear to reflect the evolutionary diversity of the particular phototrophic organism. Here we constructed a photosynthetic chimera expressing the Roseiflexus castenholzii LH and Rhodospirillum rubrum RC complexes in a photocomplex-deficient Rsp. rubrum mutant, and spectroscopy confirmed LH expression with absorption maxima at 878 and 801 nm. The chimeric strain grew slower phototrophically than wildtype but faster than a strain containing only the RC, indicating partial energy transfer from LH to RC. Cryo-EM structural analysis revealed that the Rfl. castenholzii LH independently assembled into a closed ring of 15 αβ heterodimers lacking carotenoids, resulting in a blue-shifted Q_y transition, while the Rsp. rubrum RC formed a separate complex with an RC:LH ratio of ∼17:1 instead of a typical 1:1. Structural differences, including the absence of two Rfl. castenholzii-specific small proteins, likely precluded formation of a conjoined LH-RC in the chimeric strain. These results reveal that distinct photocomplex assembly strategies exist in phylogenetically divergent species and underscore the modularity and adaptability of photosynthetic complexes, offering insights for artificial photosystem design.