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

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Abstract
The photoreaction center from R. rubrum strain G9 binds about 6 times as much sodium dodecyl sulfate [SDS] as certain proteins commonly used as MW markers for SDS-polyacrylamide gel electrophoresis. This presumably explains the apparent discrepancy between the MW of the photoreaction center determined by electrophoresis (76,000) and its minimal MW (87,000). The MW of the photoreaction center solubilized with Triton X-100 was determined by 3 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 3 methods gave MW values close to 60,000. A similar MW was found for the photoactive .beta..gamma. dimer obtained from the photoreaction center of R. sphaeroides strain 2.4.1 which, as a whole, had a MW of 87,000. The photoreaction center from R. sphaeroides is apparently an oligomer of the type .alpha.1.beta.1.gamma.1. In contrast, the photoreaction center from R. rubrum appears to be dissociated, in solution, into a photoactive .beta..gamma. dimer and a free polypeptide .alpha.