9513660 Schaefer Light is a critical environmental factor for photosynthetic organisms. In addition to driving photosynthesis, it provides the information necessary for acclimation and development in response to changes in ambient conditions. Most photosynthetic organisms can modulate their photosynthetic capacity to accommodate fluctuations in light availability. In many cases, the genes that encode components of the photosynthetic apparatus are under the control of wavelength-specific photoreceptors that initiate light-responsive signal transduction pathways. However, although photoperception and photoregulation of gene expression are well documented for different prokaryotic and eukaryotic phototrophs, the molecular mechanisms by which photoreceptors communicate with the gene regulatory machinery of cells are not. This research seeks to define a photoregulatory mechanism by focusing on photoperception and signal transduction involved in complementary chromatic adaptation by the filamentous cyanobacterium Fremyella diplosiphon. Complementary chromatic adaptation is the process by which this organism senses changes in light quality and responds by altering the phycobiliprotein composition of the light-harvesting phycobilisome. This acclimation response involves a signaling mechanism that links the activity of a photoreceptor to regulated expression of specific phycobiliprotein genes. Two molecular genetic approaches are being used to identify and isolate genes involved in complementary chromatic adaptation. Both approaches are based on a collection of pigment mutants characterized by aberrant chromatic adaptation (regulatory mutants) or altered phycobilisome structure (structural mutants). The first approach involves complementation of different classes of chromatic adaptation regulatory mutants. A complementation library of wild-type genomic DNA will be constructed and introduced into mutant cells by conjugation. For each complemented exconjugant, the complementing plasmid will be rescued an d analyzed to identify the responsible gene(s). The second approach is based on a recently developed system for identifying chromatic adaptation genes by transposon-tagging with endogenous transposon Tn5469. Select pigment mutants in which replicative transposition of Tn5469 was the probable mutagenic event have been identified and will be used to isolate the affected gene. A correlation between Tn5469 transposition and the mutant phenotype will be confirmed by complementation with a corresponding wild-type genomic DNA fragment. By characterizing the isolated genes, a molecular frarnework for the chromatic adaptation photosensory and signaling mechanisms can be established. This work offers a unique opportunity to dissect the signaling circuitry by which a photoreceptor communicates with the genes it regulates and will provide a new window into the molecular basis of biological phototransduction. *** =?
