Photochemical apparatus organization in Synechococcus 6301 (Anacystis nidulans). Effect of phycobilisome mutation

Abstract

The photochemical apparatus organization in Synechococcus 6301 (Cyanophyceae) was investigated under various experimental conditions. Wild type (WT) Synechococcus produced phycobilisomes (PBSs) containing normal levels of phycocyanin (Phc) and allophycocyanin (Aphc). The ratio of reaction centers(RC) RCII/RCI of 0.4 was the same in WT and the mutant strain AN112, whereas RCH/PBS was 1.9:1 in WT and 1:1 in AN112. Excitation of WT cells with broad-band 620 nm light, which is absorbed primarily by Phc and Aphc and to a much lesser extent by chlorophyll (Chl), sensitized the RC of photosystem (PS) II at about 15 times the rate it sensitized RCI. This implies that PBSs are associated exclusively with PSII complexes and that PBS excitation is not transferred to PSI. The AN112 mutant of Synechococcus produced smaller PBSs consisting of the Aphc-containing core and of only six Phc-containing hexamers, respectively. It lacked about 65% of the Phccontaining rod substractures. Under our experimental conditions, the effective absorption cross section of the mutant PBS was only about half that of the WT. In agreement, the rate of RCII excitation by 620 nm light was also about half of that measured in the WT. Thus, the rate of light absorption by PSII depends directly on PBS size and composition. The low rate of RCI excitation with 620 nm light was the same in WT and AN112 cells, apparently independent of the PBS effective absorption cross section. We propose a strict structural-functional association between PBS and PSII complex. PSI is a structurally distinct entity and it receives excitation independently from its own Chl light-harvesting antenna.