Rapid quenching of chlorophyll excited states in cyanobacteria, even in the absence of reaction centers

Abstract

Reaction center complexes from oxygenic photosynthetic organisms contain chlorophyll a but lack chlorophyll b. However, peripheral antennas from plants, algae, and prochlorophytes contain both chlorophylls. This differential distribution generally is attributed to chlorophyll a binding specificity of reaction center proteins. However, we found that the pigment content of chlorophyll-binding proteins is determined in part by the local availability of specific chlorophylls at the time a chlorophyll-binding protein is synthesized. When the cyanobacterium Synechocystis sp. PCC 6803 (which naturally contains chlorophyll a but lacks chlorophyll b) is provided with the gene for both chlorophyll a oxygenase (required for chlorophyll b synthesis) and LHCII, the majority of the chlorophyll in PSII core complexes is chlorophyll b; in reaction center preparations about half of the chlorophyll is chlorophyll b. We interpret this to indicate that LHCII, which is unstable in Synechocystis, is required for chlorophyll a oxygenase activity, and that synthesized chlorophyll b is used by nascent PSII complexes. Resulting PSII core complexes with more than half of the chlorophyll a molecules replaced by chlorophyll b are functionally normal. Earlier experiments suggested a similar lack of specificity with respect to carotenoid binding in Synechocystis. Therefore, most pigment binding sites are not specific for one pigment and different pigments can be functionally accommodated at a particular site. The nature of the pigment to be incorporated at a specific site is determined in part by the relative availabilities of pigments. This allows the evolution of pigments and pigment-binding proteins to be rather independent processes.