Environmental Regulation of Phycobilisome Biosynthesis

Abstract

Photosynthetic activity and the composition of the photosynthetic apparatus are strongly regulated by environmental conditions. Some of the most visually dramatic changes in pigmentation of cyanobacteria during changing nutrient and light conditions reflect marked alterations in components of the major light-harvesting complex in these organisms, the phycobilisome. In some cyanobacteria the composition of the phycobilisome is very sensitive to the wavelengths of light in the environment. The populations of the different pigmented polypeptides or phycobiliproteins, phycocyanin and phycoerythin, of the phycobilisome are adjusted to optimize absorption of excitation energy present in the environment. This process, called complementary chromatic adaptation, is controlled by a photoreceptor that binds a bilin chromophore and has some similarity to phytochrome of vascular plants. This photoreceptor is thought to represent the first element of a phosphorelay system that regulates genes encoding the phycobiliprotein subunits and linker polypeptides. Phycobilisomes are also sensitive to nutrient levels and during starvation conditions there is both reduced synthesis and elevated breakdown of phycobilisomes. The degradation of phycobilisomes during nutrient-limited growth results in cells that lose their brilliant blue-green color and appear yellow green or bleached. This bleaching response is controlled by a ‘global’ regulatory system that may sense the redox state of the cell, the generation of reactive oxygen species and the quality of light in the environment. Some of the regulatory elements critical for controlling nutrient stress responses are also involved in modulating photosynthetic activity when cyanobacteria experience high light conditions. The analyses of these systems highlight the molecular flexibility incorporated into the biosynthetic processes required for construction and maintenance of a light harvesting complex and the nature of the key control elements that interface with environmental cues. At a more basic level, these studies suggest the robustly dynamic nature of the entire photosynthetic apparatus.