The PshX subunit of the photochemical reaction center from Heliobacterium modesticaldum acts as a low-energy antenna

Gregory S. Orf, Christopher J. Gisriel, Jesse Granstrom, Patricia L. Baker, Kevin E. Redding

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


The anoxygenic phototrophic bacterium Heliobacterium modesticaldum contains a photochemical reaction center protein complex (called the HbRC) consisting of a homodimer of the PshA polypeptide and two copies of a newly discovered polypeptide called PshX, which is a single transmembrane helix that binds two bacteriochlorophyll g molecules. To assess the function of PshX, we produced a ∆pshX strain of Hbt. modesticaldum by leveraging the endogenous Hbt. modesticaldum Type I-A CRISPR-Cas system to aid in mutant selection. We optimized this system by separating the homologous recombination and CRISPR-based selection steps into two plasmid transformations, allowing for markerless gene replacement. Fluorescence and low-temperature absorbance of the purified HbRC from the wild-type and ∆pshX strains showed that the bacteriochlorophylls bound by PshX have the lowest site energies in the entire HbRC. This indicates that PshX acts as a low-energy antenna subunit, participating in entropy-assisted uphill energy transfer toward the P800 special bacteriochlorophyll g pair. We further discuss the role that PshX may play in stability of the HbRC, its conservation in other heliobacterial species, and the evolutionary pressure to produce and maintain single-TMH subunits in similar locations in other reaction centers.

Original languageEnglish (US)
Pages (from-to)11-30
Number of pages20
JournalPhotosynthesis research
Issue number1
StatePublished - Jan 2022


  • CRISPR-Cas
  • Excitonically coupled chlorophyll
  • Heliobacteria
  • Reaction center
  • Supernumerary subunits
  • uphill energy transfer

ASJC Scopus subject areas

  • Biochemistry
  • Plant Science
  • Cell Biology


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