The role of P870 in bacterial photosynthesis

Abstract

In Chromatium chromatophores, a 30-nsec flash causes an absorbance decrease at 882 nm which is interpreted as an oxidation of P870. The quantum yield is close to one. The reaction is complete in less than 0.5 μsec. It does not occur in the presence of dithionite. With a half-time of 2 μsec, the 882-nm absorbance increases again. Separate measurements show that cytochrome C422 becomes oxidized at the same rate. The 882-nm absorbance increase is interpreted as a reduction of oxidized P870 (P870 +). There appears to be a direct electron transfer between cytochrome C422 and P870 +. The 2-μsec P870 + reduction is generally incomplete at 298 °M. It does not occur at 80 °K, nor does the cytochrome oxidation occur at this temperature. At 298 °K, the addition of N-methylphenazonium methosulfate (PMS) causes the 2-μsec reduction to be complete. In the presence of PMS, the cytochrome oxidation occurs with a quantum yield near one, and its rate and extent are similar to those which occur in whole cells. In Rhodospirillum rubrum chromatophores, the P870 + reduction is much slower. In this species, the initial absorbance changes of P800 are complete within 0.2 μsec. The results support the view that P870 oxidation is the primary chemical reaction of photosynthesis in Chromatium chromatophores.