Interchangeable Copper and Iron Proteins in Algal Photosynthesis. Studies on Plastocyanin and Cytochrome c-552 in Chlamydomonas

Abstract

The interrelation of the copper plastocyanin, and a soluble c-type cytochrome, c-552, in photosynthetic electron transport has been studies in the genus Chlamydomonas. With C. reinhardtii the plastocyanin:cytochrome c-552 ration could be changes from 300:1 to < 1:16 simply by omitting copper from the medium, without any other detectable change. Plastocyanin was indetectable in a second species, C. mundana, for which the cytochrom c-552 level was always very high. The properties of Levin's C. reinhardtii mutant lacking plastocyanin, ac-208, were studies and it was foiund that the photosynthetic capabilities of a suppressed phenotype and suppressed genotype could be explained by reference to the cytochrome c-552n levels. Both proteins were successfully used in reconsitution experiments with chloroplast fragments. Both showed very fast kinetics for reduction by purified Chlamydomonas cytochrome f, but the rate of electron transfer from one to other was much slower. It is concluded that they consitute an interchangeable pari, and the rationale for this and possible analogies are both discussed.