The unicellular cyanobacterium Synechocystis sp. strain 6701 was mutagenized with UV irradiation and screened for pigment changes that indicated genetic lesions involving the light-harvesting proteins of the phycobilisome. A previous examination of the pigment mutant UV16 showed an assembly defect in the phycocyanin component of the phycobilisome. Mutagenesis of UV16 produced an additional double mutant, UV16-40, with decreased phycoerythrin content. Phycocyanin and phycoerythrin were isolated from UV16-40 and compared with normal biliproteins. The results suggested that the UV16 mutation affected the a subunit of phycocyanin, while the phycoerythrin subunit from UV16-40 had lost one of its three chromophores. Characterization of the unassembled phycobilisome components in these mutants suggests that these strains will be useful for probing in vivo the regulated expression and assembly of phycobilisomes.Phycobilisomes are the light-harvesting structures of cyanobacteria and red algae. Composed of chromophoric biliproteins and nonchromophoric linker proteins, phycobilisomes are organized into two structural domains, the core and the rods (reviewed in reference 12). The core is associated with the thylakoid membrane and contains the allophycocyanin (AP) biliproteins. The rods are attached to the membrane-distal periphery of the core and, in the unicellular cyanobacterium Synechocystis sp. strain 6701, contain phycocyanin (PC) and phycoerythrin (PE) biliproteins and four linker proteins of 27, 30.5, 31.5, and 33.5 kilodaltons (kDa) (9, 12).UV irradiation has been used to obtain pigment mutants in strain 6701 (3). Preliminary characterization of a phycobilisome assembly mutant, UV16, demonstrated intact core substructures with little PC bound and soluble PC trimers that were free of linker proteins. The PE in UV16 assembled into rod substructures that contained the 31.5-and 30.5-kDa PE linker proteins and cosedimented with cores on a sucrose gradient (3). The UV16 strain was mutagenized to find a mutant that would eliminate PE contamination of cores. We obtained the mutant strain UV16-40 from a colony with lighter green pigmentation, indicating decreased PE relative to the parent strain, UV16. We examined relative biliprotein content, assembly of rod components, and the chromatic adaptation response (22) in the UV16 and UV16-40 strains. The defective PC and PE biliproteins were isolated from UV16-40 and compared with normal PC and PE from wildtype (WT) and UV16 strains. The resulting differences suggested the presence of structural lesions in the PC ax subunit of UV16 and the PE ,B subunit of UV16-40. MATERIALS AND METHODSStrains, media, and culture conditions. Synechocystis sp. strain 6701 (Pasteur Culture Collection) and mutant strains were grown in the medium and light conditions described previously (3). Strains UV16 and UV16-40 were obtained after UV mutagenesis (3) tively. The spontaneous reversion frequency for both mutants was not measured directly, but is less than lo-5. To assay chromatic adaptation respo...

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Nostoc sp. strain MAC cyanobacteria were green in color when grown in white light at 30°C and contained physobilisomes that had phycoerythrin and phycocyanin in a molar ratio of 1:1. Cells grown for 4 to 5 days in green light at 30°C or white light at 39°C turned brown and contained phycoerythrin and phycocyanin in a molar ratio of greater than 2:1. In addition to the change in pigment composition, phycobilisomes from brown cells were missing a 34.5-kilodalton, rod-associated peptide that was present in green cells. The green light-induced changes were typical of the chromatic adaptation response in cyanobacteria, but the induction of a similar response by growth at 39°C was a new observation. Phycobilisomes isolated in 0.65 M phosphate buffer (pH 7) dissociate when the ionic strength or pH is decreased. Analysis of the dissociation products from Nostoc sp. phycobilisomes suggested that the cells contained two types of rod structures: a phycocyanin-rich structure that contained the 34.5-kilodalton peptide and a larger phycoerythrin-rich complex. Brown Nostoc sp. cells that lacked the 34.5-kilodalton peptide also lacked the phycocyanin-rich rod structures in their phycobilisomes. These changes in phycobilisome structure were indistinguishable between cells cultured at 39°C in white light and those cultured at 30°C in green light. A potential role is discussed for rod heterogeneity in the chromatic adaptation response.