Photoregulation of Chlorplast Synthesis and Replication.
The synthesis and replication of cellular organelles involve an interplay between nuclear andorganellar genomes. In the case of chloroplasts the interaction may involve three genomes:nuclear,chloroplastic and mitochondrial. The mechanisms by which these genomes regulate chlorplast synthesis and replicaton is the subject of our investigations.
Euglena gracilis possesses a chloroplast developmental and division system similar in several ways to that of higher plants while also having the manipulative advantages of a microorganism: rapid growth of large populations of uniform cells, rapid mutant selection and isolation, and ease of biochemical,biophysical and microscopic aanalysis. Light is required to initiate and maintain the synthesis of the chloroplast from its precursor, the proplastid. We have identified three different light-absorbing systems in Euglena which regulate the assembly of the organelle. One of these photomorphogenic systems acts to form the second messenger, Inositol-3- phosphate which ,in turn,releases Calcium from internal stores. Calcium, binding to Calcium-binding proteins acts at a post-translational step in chloroplast assembly. The other two systems apparently act at transcriptional and enhancing steps.
Light also acts to regulate the synthesis and amount of chloroplast DNA. Evidence so far suggests that light-activated DNAses serve to regulate synthesis and amount of plastid DNA. Regulation of Phototaxis Light also controls the swimming of Euglena toward the light source. (Positive Phototaxis). Euglena possesses an eyespot containing rhodopdsin- like pigments which are also found in the paraflaggelar body. We have found using aplastidic and photosynthetic mutants that positive phototaxis requires a “signal” from a functioning chloroplast. Blocking photsynthetic electron transport with inhibitors or by mutation does not inhibit light-activated photkinesis(random swimming), but does block positive phototaxis. The molecular mechanisms underlying this are under investigation. Photoregulation of Chloroplast Synthesis and Replication. Regulation of Phototaxis Photoregulated Transmambrane Signalling