Structural analysis of the photosynthetic reaction center from the green sulfur bacterium Chlorobium tepidum

Abstract

The reaction center (RC) core complex was isolated from the green sulfur bacterium Chlorobium tepidum and characterized by gel electrophoresis, gel filtration, and analytical ultracentrifugation. The purified complex contained the PscA and PscC subunits and a small amount of the Fenna–Matthews–Olson protein (FMO protein) as an impurity. The mass of the core complexes was found to be 248 kDa by scanning transmission electron microscopy (STEM). This is compatible with the presence of two copies of the PscA subunit and at least one copy of the PscC subunit, and provides evidence that the isolated complex exists as a monomer. Digital images of negatively stained RC complexes were recorded by STEM and analyzed by single-particle averaging. The complex had a length of 14 nm and a width of 8 nm, comparable to the length and width of the monomeric cyanobacterial PSI complex. The averages revealed a pseudo two-fold symmetry axis, which is a prominent structural element of the monomeric form.