Analysis of signal transduction components in light induction of thegsa gene for an early chlorophyll biosynthetic step in Chlamydomonas reinhardtii

Light is one of the most important environmental factors for plants since it contains not only an energy source for plants but also a cue for the development and gene expression of plants. Therefore, the ability of plants to adjust themselves to different light quality and quantity is critical for their survival. I studied Chlamydomonas reinhardtii as a model system to investigate the ability of plants to adjust to the change of light environment, especially, how plants regulate chlorophyll biosynthesis, which is an energy-costing process in plants. Previously, the genes encoding two of the chlorophyll biosynthetic enzymes, glutamate-l-semialdehyde aminotransferase (GSAT) and aminolevulinic acid dehydratase (ALAD) had been cloned from Chlamydomonas reinhardtii and their expression had been shown to be induced by blue light in synchronized cultures (Matters and Beale, 1994a, 1994b, and 1995). In this study, the blue-light induced signal transduction pathway involved in GSAT expression was analyzed from receptor(s) to gene expression using biochemical and pharmacological approaches. The evidence suggests that (1) gsa expression in Chlamydomonas reinhardtfi requires nitrogen and a carbon source in the medium; (2) a flavin-based photoreceptor is involved in blue-light induction of gsa expression; (3) upon activation of the blue-light receptor, cytoplasmic [Ca2+] is increased either by taking up Ca2+ from the medium or by releasing Ca2+ from internal stores; (4) an increase in cytoplasmic inositol-3,4,5-trisphosp hate (IP3) caused by heterotrimeric G-protein-activated phospholipase C induces the internal [Ca2+] increase; (5) the increased [Ca2+] transmits the signal through calmodulin and Ca 2+/calmodulin-dependent kinase 11 rather than calcineurin; (6) protein tyrosine kinase(s), other protein kinase(s), and phosphoprotein phosphatase(s) are involved in the signal transduction pathway; (7) Ca 2+-ATPase and mitochondria are involved in the regulation of cytoplasmic [Ca2+] upon light stimulation.