Correlation of the photosystem I and II reaction center chlorophyll‐protein complexes, CP‐a1 and CP‐aII with photosystem activity and low temperature fluorescence emission properties in mutants of Scenedesmus

Abstract

Comparative studies on the low temperature fluorescence emission of whole cells, purified chlorophyll‐protein (CP) complexes and on patterns noted in sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS‐PAGE) for chlorophyll‐protein complexes and chloroplast membrane polypeptides of Scenedesmus obliquus with several distinct mutant classes has allowed further correlation between the fluorescence emission bands seen at 77K and the known chlorophyll‐protein complexes. In mutants deficient in photosystem II (PS‐II; total loss of the reducing side) the fluorescence emission spectra showed only two peaks, i.e., 686 and 718 nm, but in the wild type, in mutants lacking the oxidizing side of PS‐II and in phenotypes missing the CP‐a₁ complex (and P‐700 activity) all three emission bands at 686, 696 and 718 nm were present. In a mutant lacking the light‐harvesting CP‐a/b complex the emission peak at 686 nm was strongly reduced and the longer wavelength emissions predominated. Gel electrophoresis studies showed that the PS‐II (reducing side) mutants lacked the polypeptides of apparent molecular weight 54 and 51 kilodaltons and the chlorophyll‐protein complex, CP‐a_II, of apparent molecular weight 32 kilodaltons. Contrarily, the loss of the oxidizing side of PS‐II did not result in any alteration of these components. Genetic deletion of CP‐a₁ did not alter significantly the long wavelength emission even though the isolated CP‐a₁ shows the low temperature‐dependent long wavelength emission comparable to that seen in the whole cell. It was deduced that remaining PS‐I antennae chlorophylls must account for the emission seen at 718 nm. The absence of the CP‐a/b complex and the strong simultaneous decrease of the 686 nm emission strengthens the concept that this complex is the primary emitter of fluorescence at room temperature. Its absence facilitated the detection of the CP‐a_II complex in SDS‐PAGE and enhanced the in vivo fluorescence by the two photosystems. Parallel experiments with two mutants which green and develop, one to the wild‐type and the other to the CP‐a/b deficient phenotype, provided additional evidence for the source of the low temperature emission bands.