20 Chloroplast signal transduction in heterosigma akashiwo

Maintenance of homeostasis requires that an organism perceive selected physical and chemical signals and convert this information via signal transduction pathways into an appropriate metabolic response. The Heterosigma chloroplast genome encodes the regulatory protein of a His/Asp signal transduction system. This two‐ component system, which is also found in prokaryotic cells, allows the organism to selectively modify chloroplast gene transcription in response to environmental cues. Using artificially created 50 base oligomeres in a TRG1‐DNA binding assay, it was determined that this regulatory protein recognized the 6‐base consensus sequence TTAGGT. Analysis of 27 sequenced Heterosigma chloroplast genes revealed that this six ‐base motif was found almost exclusively in genes associated with photosystem II, Calvin cycle and energy generation functions with high statistical probability‐ suggesting that TRG1 may be a global regulator of this gene set. Molecular modeling has shown that the Heterosigma TRG1 protein with the sigma 70 subunit of eubacterial‐like RNA polymerase which we know from sequencing data to be chloroplast localized in Heterosigma. Antibodies generated against TRG1 showed no change in the concentration of this protein when vegetative Heterosigma cells were maintained on a 12 h light:12 h dark photoperiod or during resting cell biogenesis. This observation suggests that TRG1, like its bacterial counterparts, is regulated by phosphorylation. The presence of a His/Asp two‐component signal transduction pair in the chloroplast genome is taxon‐specific.