Photoreaction center of photosynthetic bacteria. 2. Size and quaternary structure of the photoreaction centers from Rhodospirillum rubrum strain G9 and from Rhodopseudomonas sphaeroides strain 2.4.1.

The photoreaction center from Rhodospirillum rubrum strain G9 binds about 6 times as much sodium dodecyl sulfate as certain proteins commonly used as molecular weight markers for sodium dodecyl sulfate--polyacrylamide gel electrophoresis. This presumably explains the apparent discrepancy between the molecular weight of the photoreaction center determined by electrophoresis (76 000) and its minimal molecular weight (87 000). The molecular weight of the photoreaction center solubilized with Triton X-100 was determined by three different methods: conventional sedimentation equilibrium, a combination of sedimentation velocity and gel filtration measurements, and sedimentation equilibrium in H2O and in D2O. Each technique required a determination of the amount of bound detergent. All three methods gave molecular weight values close to 60 000. A similar molecular weight was found for the photoactive beta gamma dimer obtained from the photoreaction center of Rhodopseudomonas sphaeroides strain 2.4.1 which, as a whole, had a molecular weight of 87 000. These results indicate that the photoreaction center from Rp. sphaeroides is an oligomer of the type alpha 1 beta 1 gamma 1. In contrast, the photoreaction center from Rs. rubrum appears to be dissociated, in solution, into a photoactive beta gamma dimer and a free polypeptide alpha.