Functional Comparison of the Photosystem II Center-Antenna Complex of a Phycocyanin-less Mutant of Cyanidium caldarium with That of Chlorella pyrenoidosa.

The photosystem II (PSII) antenna of a phycocyanin-less mutant (III-C) of Cyanidium caldarium was studied by means of O(2) activation and fluorescence induction measurements and compared to that of the green alga Chlorella pyrenoidosa.In Cyanidium this antenna contains only about 40 (Chl) a molecules per center yet shows PSII energy transfer as efficient as in Chlorella with 240 Chl a + b per center. In Cyanidium, PSII energy transfer occurs among at least four centers which are located close to each other. Cyanidium and Chlorella show the same PSII action and low temperature emission spectra, in the Chl a region despite the gross differences in antenna size.It is proposed that the PSII center-antenna complex found in Cyanidium is a core unit of the center-antenna complex of green algae; the only major difference between them is the addition of light-harvesting Chl a/b in the latter. It is further proposed that this core is the site of energy transfer between PSII centers.