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.     

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.     

Characterization of complementary chromatic adaptation in Gloeotrichia UTEX 583 and identification of a transposon-like insertion in the cpeBA operon

Many cyanobacteria are able to alter the pigment composition of the phycobilisome in a process called complementary chromatic adaptation (CCA). The regulatory mechanisms of CCA have been identified in Fremyella diplosiphon, which regulates both phycoerythrin and phycocyanin levels, and Nostoc punctiforme, which regulates only phycoerythrin production. Recent studies show that these species use different regulatory proteins for CCA. We chose to study the CCA response of Gloeotrichia UTEX 583 in an effort to expand our knowledge about CCA and its regulation. We found that Gloeotrichia 583 has a CCA pigment response more similar to that of N. punctiforme rather than F. diplosiphon and exhibits none of the CCA-regulated morphological responses seen in F. diplosiphon. Preliminary experiments suggest that Gloeotrichia 583 contains a homolog to the CCA photoreceptor from N. punctiforme but not the CCA photoreceptor from F. diplosiphon. Additionally, two spontaneous mutants lacking phycoerythrin production were identified. Analysis has shown that these mutants contain a transposon-like insertion in the cpeA gene, which encodes the α subunit of phycoerythrin. These results suggest that CCA in Gloeotrichia UTEX 583 is more similar to that of N. punctiforme than it is to F. diplosiphon, a closely related species.     

The photoregulated expression of multiple phycocyanin species. A general mechanism for the control of phycocyanin synthesis in chromatically adapting cyanobacteria

The regulation of phycocyanin synthesis in response to growth in chromatic illumination was studied in 69 strains of cyanobacteria. Cyanobacteria (24 of 31 strains examined), which chromatically adapt by modulating the synthesis of both phycocyanin and phycoerythrin, controlled phycocyanin synthesis through the differential, photoregulated expression of two phycocyanin species (two alpha-type and two beta-type subunits). For these strains the expression of one pair of phycocyanin subunits was constitutive (i.e. irrespective of the light wavelength in which the cells were grown); the expression of the second pair of phycocyanin subunits occurred specifically during growth in red light. Two facultatively heterotrophic cyanobacteria, Calothrix strains 7101 and 7601, synthesized both the constitutive and the inducible pairs of phycocyanin subunits when grown heterotrophically in the dark after transfer from either red or green light. No evidence for the existence of multiple and/or photoregulated phycocyanin species was found for cyanobacteria (25 strains) incapable of chromatic adaptation, nor for cyanobacteria (13 strains) which chromatically adapt by modulating the synthesis of phycoerythrin alone.     

Phycobilisome structure and function

Phycobilisomes are aggregates of light-harvesting proteins attached to the stroma side of the thylakoid membranes of the cyanobacteria (blue-green algae) and red algae. The water-soluble phycobiliproteins, of which there are three major groups, tetrapyrrole chromophores covalently bound to apoprotein. Several additional protiens are found within the phycobilisome and serve to link the phycobiliproteins to each other in an ordered fashion and also to attach the phycobilisome to the thylakoid membrane. Excitation energy absorbed by phycoerythrin is transferred through phycocyanin to allophycocyanin with an efficiency approximating 100%. This pathway of excitation energy transfer, directly confirmed by time-resolved spectroscopic measurements, has been incorporated into models describing the ultrastructure of the phycobilisome. The model for the most typical type of phycobilisome describes an allophycocyanin-containing core composed of three cylinders arranged so that their longitudinal axes are parallel and their ends form a triangle. Attached to this core are six rod structures which contain phycocyanin proximal to the core and phycoerythrin distal to the core. The axes of these rods are perpendicular to the longitudinal axis of the core. This arrangement ensures a very efficient transfer of energy. The association of phycoerythrin and phycocyanin within the rods and the attachment of the rods to the core and the core to the thylakoid require the presence of several linker polypeptides. It is recently possible to assemble functionally and structurally intact phycobilisomes in vitro from separated components as well as to reassociate phycobilisomes with stripped thylakoids. Understanding of the biosynthesis and in vivo assembly of phycobilisomes will be greatly aided by the current advances in molecular genetics, as exemplified by recent identification of several genes encoding phycobilisome components.Combined ultrastructural, biochemical and biophysical approaches to the study of cyanobacterial and red algal cells and isolated phycobilisome-thylakoid fractions are leading to a clearer understanding of the phycobilisome-thylakoid structural interactions, energy transfer to the reaction centers and regulation of excitation energy distribution. However, compared to our current knowledge concerning the structural and functional organization of the isolated phycobilisome, this research area is relatively unexplored.     

Complementary chromatic adaptation: photoperception to gene regulation

Many photosynthetic organisms can acclimate to the quantity and quality of light present in their environment. In certain cyanobacteria the wavelengths of light in the environment control the synthesis of specific polypeptides of light harvesting antenna complex or phycobilisome. This phenomenon, called complementary chromatic adaptation, is most dramatically observed in comparison of cyanobacteria after growth in green light and red light. In red light-grown cells the phycobilisome is largely composed of phycocyanin and its associated linker polypeptides (the latter are important for the assembly of the phycocyanin subunits and their placement within the light harvesting structure); the organisms appear blue-green color. In green light-grown cells the phycobilisome is largely composed of phycoerythrin and its associated linker polypeptides; the organisms appear red in color. The ways in which these cyanobacteria sense their changing light environment and the regulatory elements involved in controlling the process of complementary chromatic adaptation are discussed in this review.     

Heterologous assembly and rescue of stranded phycocyanin subunits by expression of a foreign cpcBA operon in Synechocystis sp. strain 6803.

Light harvesting in cyanobacteria is performed by the biliproteins, which are organized into membrane-associated complexes called phycobilisomes. Most phycobilisomes have a core substructure that is composed of the allophycocyanin biliproteins and is energetically linked to chlorophyll in the photosynthetic membrane. Rod substructures are attached to the phycobilisome cores and contain phycocyanin and sometimes phycoerythrin. The different biliproteins have discrete absorbance and fluorescence maxima that overlap in an energy transfer pathway that terminates with chlorophyll. A phycocyanin-minus mutant in the cyanobacterium Synechocystis sp. strain 6803 (strain 4R) has been shown to have a nonsense mutation in the cpcB gene encoding the phycocyanin beta subunit. We have expressed a foreign phycocyanin operon from Synechocystis sp. strain 6701 in the 4R strain and complemented the phycocyanin-minus phenotype. Complementation occurs because the foreign phycocyanin alpha and beta subunits assemble with endogenous phycobilisome components. The phycocyanin alpha subunit that is normally absent in the 4R strain can be rescued by heterologous assembly as well. Expression of the Synechocystis sp. strain 6701 cpcBA operon in the wild-type Synechocystis sp. strain 6803 was also examined and showed that the foreign phycocyanin can compete with the endogenous protein for assembly into phycobilisomes.     

A Polypeptide with Similarity to Phycocyanin α-Subunit Phycocyanobilin Lyase Involved in Degradation of Phycobilisomes

To optimize the utilization of photosynthate and avoid damage that can result from the absorption of excess excitation energy, photosynthetic organisms must rapidly modify the synthesis and activities of components of the photosynthetic apparatus in response to environmental cues. During nutrient-limited growth, cyanobacteria degrade their light-harvesting complex, the phycobilisome, and dramatically reduce the rate of photosynthetic electron transport. In this report, we describe the isolation and characterization of a cyanobacterial mutant that does not degrade its phycobilisomes during either sulfur or nitrogen limitation and exhibits an increased ratio of phycocyanin to chlorophyll during nutrient-replete growth. The mutant phenotype was complemented by a gene encoding a polypeptide with similarities to polypeptides that catalyze covalent bond formation between linear tetrapyrrole chromophores and subunits of apophycobiliproteins. The complementing gene, designated nblB, is expressed at approximately the same level in cells grown in nutrient-replete medium and medium devoid of either sulfur or nitrogen. These results suggest that the NblB polypeptide may be a constitutive part of the machinery that coordinates phycobilisome degradation with environmental conditions.