Identification of possible signal transduction components mediating light induction of the Gsa gene for an early chlorophyll biosynthetic step in Chlamydomonas reinhardtii

Abstract.

 Light-induced expression of the Gsa gene encoding the heme and chlorophyll biosynthetic enzyme glutamate 1-semialdehyde aminotransferase in Chlamydomonas reinhardtii was previously shown to involve Ca²⁺ and calmodulin (CaM) (C. lm et al. 1996, Plant Cell 8: 2245–2253). To further analyze the signal transduction pathway for light-induced Gsa expression, the effects of several pharmacological agents were examined. Treatment of light-dark synchronized cells with the heterotrimeric G-protein agonist Mas-7 caused partial induction of Gsa in the dark. The phospholipase C inhibitor U73122 inhibited light induction of Gsa. Exposure of cells to light caused a sustained 3-fold increase in cellular d-inositol 1,4,5-trisphosphate (InsP₃) concentration. KN-93, a specific inhibitor of Ca²⁺/CaM-dependent protein kinase II, inhibited light induction of Gsa. In contrast, cyclosporin A, a specific inhibitor of the Ca²⁺/CaM-dependent phosphoprotein phosphatase calcineurin, did not affect light induction of Gsa. These results, together with the earlier results, suggest the involvement of a canonical signal transduction pathway for light-regulated Gsa expression that involves a heterotrimeric G-protein activation, phospholipase C-catalyzed InsP₃ formation, InsP₃-dependent Ca²⁺ release, and activation of a downstream signaling pathway through a Ca²⁺/CaM-dependent protein kinase.