Genes encoding phycobilisome linker polypeptides on the plastid genome of Aglaothamnion neglectum (Rhodophyta)

The genes encoding the phycobilisome anchor protein (apcE) and rod-core linker (cpcG) are on the plastid genome of the red alga Aglaothamnion neglectum. The apcE gene product is 5 to and in the same operon as the and subunit genes of allophycocyanin. This arrangement is identical to the arrangement observed in many cyanobacteria. The cpcG gene product is 5 to the operon encoding the and subunits of phycoerythrin, but is transcribed from the opposite DNA strand. This gene arrangement is different from that observed in cyanobacteria.The amino acid sequences of the A. neglectum anchor protein and rod-core linker polypeptide, as deduced from the nucleotide sequences of the genes, are approximately 50% identical to analogous polypeptides from cyanobacteria and another eukaryotic alga Cyanophora paradoxa. The conserved nature of these proteins suggests that the structure of the core and the rod-core interface are very similar in phycobilisomes of cyanobacteria and eukaryotic red algae.     

Characterization and transcript analysis of the major phycobiliprotein subunit genes from Aglaothamnion neglectum (Rhodophyta)

The genes encoding the and subunits of allophycocyanin, phycocyanin and phycoerythrin from the red alga Aglaothamnion neglectum were isolated and characterized. While the operons containing the different phycobiliprotein genes are dispersed on the plastid genome, the genes encoding the and subunits for each phycobiliprotein are contiguous. The subunit gene is 5 for both the phycocyanin and phycoerythrin operons, while the subunit gene is 5 for the allophycocyanin operon. The amino acid sequences of A. neglectum phycobiliproteins, as deduced from the nucleotide sequences of the genes, are 65–85% identical to analogous proteins from other red algae and cyanobacteria. The conserved nature of the plastid-encoded red algal and cyanobacterial phycobiliprotein genes supports the proposed origin of red algal plastids from cyanobacterial endosymbionts.Many environmental factors effect phycobilisome biosynthesis. The effect of both nutrient availability and light quantity on the level of A. neglectum phycobiliprotein subunits and the mRNA species encoding those subunits is described.     

Chromatic adaptation and the events involved in phycobilisome biosynthesis

Abstract. The major light-harvesting complex in cyanobacteria and red algae is the phycobilisome, a macromolecular complex that is attached to the surface of the photosynthetic membranes. The phycobilisome is composed of a number of different chromophoric polypeptides called phycobiliproteins and nonchromophoric polypeptides called linker proteins. Several environmental parameters modulate the synthesis, assembly and degradation of phycobilisome components. In many cyanobacteria, the composition of the phycobilisome can change to accommodate the prevalent wavelengths of light in the environment. This phenomenon is called complementary chromatic adaptation. Organisms that exhibit complementary chromatic adaptation must perceive the wavelengths of light in the environment and transduce the light signals into a sequence of biochemical events that result in altering the activities of genes encoding specific phycobiliprotein and linker polypeptides. Other environmental parameters such as light intensity and nutrient status can also have marked effects on both the number and composition of the phycobilisomes. The major concern of this article is the molecular events involved in chromatic adaptation. Most of the information concerning this process has been gained from studies involving the filamentous cyanobacterium Fremyella diplosiphon . However, also briefly considered are some of the complexities involved in phycobilisome biosynthesis and degradation; they include post-translational modification of phycobilisome polypeptides, the coordinate expression of chromophore and apobiliprotein, the specific degradation of phycobilisomes when cyanobacteria are deprived of macronutrients such as nitrogen, sulphur and phosphorus, and the assembly of the individual phycobilisome components into substructures of the light harvesting complex.     

Phycobilin/chlorophyll excitation equilibration upon carotenoid-induced non-photochemical fluorescence quenching in phycobilisomes of the cyanobacterium Synechocystis sp. PCC 6803

To determine the mechanism of carotenoid-sensitized non-photochemical quenching in cyanobacteria, the kinetics of blue-light-induced quenching and fluorescence spectra were studied in the wild type and mutants of Synechocystis sp. PCC 6803 grown with or without iron. The blue-light-induced quenching was observed in the wild type as well as in mutants lacking PS II or IsiA confirming that neither IsiA nor PS II is required for carotenoid-triggered fluorescence quenching. Both fluorescence at 660 nm (originating from phycobilisomes) and at 681 nm (which, upon 440 nm excitation originates mostly from chlorophyll) was quenched. However, no blue-light-induced changes in the fluorescence yield were observed in the apcE⁻ mutant that lacks phycobilisome attachment. The results are interpreted to indicate that interaction of the Slr1963-associated carotenoid with - presumably - allophycocyanin in the phycobilisome core is responsible for non-photochemical energy quenching, and that excitations on chlorophyll in the thylakoid equilibrate sufficiently with excitations on allophycocyanin in wild type to contribute to quenching of chlorophyll fluorescence.     

Role of the PB-loop in ApcE and phycobilisome core function in cyanobacterium Synechocystis sp. PCC 6803

The phycobilisome (PBS) is a giant highly-structured pigment-protein antenna of cyanobacteria and red algae. PBS is composed of the phycobiliproteins and several linker polypeptides. The large core-membrane linker protein (L_CM or ApcE) influences many features and functions of PBS and consists of several domains including the chromophorylated PB-domain. Being homologous to the phycobiliprotein α-subunits this domain includes a so-called PB-loop insertion whose functions are still unknown. We have created the photoautotrophic mutant strain of the cyanobacterium Synechocystis sp. PCC 6803 with lacking PB-loop. Using various spectral techniques we have demonstrated that this mutation does not destroy the PBS integrity and the internal PBS excitation energy transfer pathways. At the same time, the deletion of the PB-loop leads to the decrease of connectivity between the PBS and thylakoid membrane and to the compensatory increase of the relative photosystem II content. Mutation provokes the violation of the thylakoid membranes arrangement, the inability to perform state transitions, and diminishing of the OCP-dependent non-photochemical PBS quenching. In essence, even such a minute mutation of the PBS polypeptide component, like the PB-loop deletion, becomes important for the concerted function of the photosynthetic apparatus.     

Transposon insertion in genes coding for the biosynthesis of structural components of the Anabaena sp. phycobilisome

Anabaena sp. PCC 7120 mutants defective in phycobiliprotein biosynthesis or phycobilisome assembly were generated by transposon mutagenesis. Four mutants with grossly reduced content of the major phycobiliprotein, phycocyanin, were found to have insertions within the cpcBACDEFG1G2G3G4 operon coding for phycocyanin biosynthesis and assembly. The insertion in mutant B646 separated the promoter from the open reading frames and eliminated production of the phycocyanin (CpcA) and (CpcB) subunits. Insertion in cpcC in mutant B642 eliminated production of the L³⁶ _Rlinker polypeptide required for assembly of phycocyanin into the distal discs of the phycobilisome rod substructures. Mutants B64328 and B64407 had insertions, respectively, in cpcE and cpcF, genes coding for the subunits of the heterodimeric lyase which catalyzes the attachment of phycocyanobilin to the phycocyanin apo- subunit. Mutant SB12, often unable to survive under low light, was found to have an insertion in the apcE gene coding for the large core-membrane linker (L¹²⁸ _CM) that provides the scaffold for assembly of the phycobilisome core. DNA sequencing 3 of apcE revealed genes apcABC, coding for the and subunits of allophycocyanin and for the small core linker L^7.8 _C. Amino acid sequence comparisons showed that the ApcA and ApcB proteins are 37% identical and that each of these polypeptides is highly similar to corresponding polypeptides from the distantly related filamentous strains Calothrix sp. PCC7601 and Mastigocladus laminosus.     

The atp1 and atp2 operons of the cyanobacterium Synechocystis sp. PCC 6803

The two operons atp1 and atp2, encoding the subunits of the F_OF₁ ATP-synthase, have been cloned and sequenced from the cyanobacterium Synechocystis sp. PCC 6803. The organization of the different genes in the operons have been found to resemble that of the cyanobacteria Synechococcus sp. PCC 6301 and Anabaena sp. PCC 7120. The Synechocystis F_OF₁ ATP-synthase has nine subunits. A tenth open reading frame with unknown function was detected at the 5 end of atp1, coding for a putative gene product similar to uncI in Escherichia coli.A promoter structure was inferred for the Synechocystis atp operons and compared to other known promoters of cyanobacteria. Even though the operon structure of atp1 and atp2 in Synechocystis resembles the corresponding operons of Synechococcus, the amino acid sequences of individual gene products show marked differences. Genetic distances between cyanobacterial genes and genes for ATP-synthase subunits from other species have been calculated and compiled into evolutionary trees.