Variation in the polypeptide composition of phycobilisomes from Anacystis nidulans and three pigment mutants

Abstract

Phycobilisomes, light harvesting antenna pigment systems, were studied from Anacystis nidulans wild type and from several spontaneous pigment mutants selected for improved growth in far-red light (>650 nm). This is the first characterization and description of polypeptide composition of phycobilisomes from spontaneous mutants (not chemically induced) of A. nidulans. The mutants had significant changes in the phycobiliprotein content relative to chlorophyll (Chl). Two phycobiliproteins, C-phycocyanin (λ_max 625 nm) and allophycocyanin (λ_max 650 nm) were present in a molar ratio of ∼3:1 in the wild type. In the mutants the amount of allophycocyanin (APC) per cell remained constant but the phycocyanin (PC) content varied. Phycobilisomes of the mutants 85Y, 19Y and 59G contained PC and APC in a molar ratio of 0.4:1, 0.7:1, and 1:1, respectively. Even though the phycobilisomes of mutant 85Y showed the greatest reduction in PC and consisted mostly of the APC core, the phycobilisomes were still functional and the growth rate of the 85Y cells was similar to the wild type. Fluorescence emission maxima of wild type and all the mutants were at 680 nm (23°C). Excitation maxima corresponded closely with expectations from absorption spectra. On SDS-PAGE gradient gels wild-type phycobilisomes had the usual 17–19 kD polypeptides of PC and APC, plus polypeptides at ca. 78, 56, 39, 36 and 31 kD. Mutants 19Y and 85Y, with the greatest PC deficiency were also deficient in the 36 and 39 kD polypeptides, presumably due to a decrease in peripheral rod structures. Energetically functional hybrid phycobilisomes were obtained when complexes of phycoerythrin-phycocyanin from Nostoc were mixed with dissociated phycobilisomes of wild-type and mutant 85Y. The 31 kD polypeptide is common to both species and immunologically cross reactive.