Characterization of light-harvesting mutants of Rhodopseudomonas sphaeroides. I. Measurement of the efficiency of energy transfer from light-harvesting complexes to the reaction center.

Light-harvesting mutants of Rhodopseudomonas sphaeroides lacking either the B800-850 complex or the B875 complex have been characterized by their absorption spectra in the visible and near-infrared region, and by their ability to transfer energy from the light-harvesting complexes to the reaction center. A new method of measuring the relative efficiency of energy transfer from the light-harvesting complexes to the reaction center is described. The B875- mutant had absorption maxima in the near-infrared at 800 and 849 nm with no evidence of an 875-nm shoulder. The efficiency of energy transfer from the light-harvesting complexes to the reaction center in the B875- mutant was 24% of the value measured for the wild-type strain and the B800-850- mutant. Yet, despite the fact that the efficiency of energy transfer for the B800-850- mutant and the wild-type strain were the same, there was a large difference in their photosynthetic unit size. These results are discussed in the context of a model in which light energy captured by the B800-850 complexes is transferred through the B875 complexes to the reaction center.