Genetic Analysis of Complementary Chromatic Adaptation

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95-13576 Grossman The phycobilisome, a light-harvesting complex in both eukaryotic red algae and prokaryotic cyanobacteria, can constitute up to 40% of the cellular protein. A number of filamentous cyanobacteria can acclimate to different wavelengths of light by altering the polypeptide and chromophore composition of the phycobilisome, thereby making the complex more effective in absorbing the prevalent wavelengths of light in the environment; this process is called complementary chromatic adaptation. We have isolated several mutants of the filamentous cyanobacterium Fremyella diplosiphon that exhibit aberrant complementary chromatic adaptation. Complementation of one of these mutants (red mutant or FdR) led to the isolation of rcaC, a gene that encodes a key regulator of chromatic adaptation. RcaC has similarities to regulators of two component regulatory systems, although it has two rather than one receiver domain (which contains the aspartate residue important for the function of the protein). The main goal of the project is to elucidate the mechanisms that govern chromatic adaptation by identifying the photoreceptor(s) and components of the light-responsive signal transduction chain. The photoreceptor(s) and some of the elements of the signal transduction chain are likely to bear functional and evolutionary relationships to photoregulatory systems in eukaryotic algae and land plants. Specifically, we will complete the characterization (both wavelength-and intensity-dependent responses) of strains in which RcaC has been altered in each or both of the aspartates of the receiver domains. Furthermore, we will characterize the genes that complement additional strains that have aberrant chromatic adaptation. One mutant that exhibits no complementary chromatic adaptation has been complemented by a gene that encodes a protein that has some similarity to phytochrome. This suggest that the phytochrome photoreceptor of higher plants may have evolved from cyanobacterial photoreceptors. Analy ses of the genes that complement mutants in chromatic adaptation should enable us to define elements of the signal transduction chain and the ways in which the element interact. Finally, we will continue to use both in vivo and in vitro methods to analyze the promoter region of the red light-activated cpcB2A2 gene set (encoding phycocyanin polypeptides) and the green light-activated cpeBA gene set (encoding phycoerythrin polypeptides), and are developing strategies to define both cis and trans acting factors that are involved in controlling cellular responses to both light quality and intensity during chromatic adaptation. %%% The research will help us understand how light affects the growth and development of photosynthetic organisms. An understanding of how light modifies processes in photosynthetic organisms may enable us to tailor plants to specific light environments. ***