Calcium and Calmodulin Are Involved in Blue Light Induction of the gsa Gene for an Early Chlorophyll Biosynthetic Step in Chlamydomonas

The Chlamydomonas reinhardtii nuclear gene gsa, which encodes the early chlorophyll biosynthetic enzyme glutamate 1-semialdehyde aminotransferase (GSAT), is specifically induced by blue light in cells synchronized in a 12-hr-light and 12-hr-dark regime. Light induction required the presence of a nitrogen source in the incubation medium. Maximal induction also required acetate. However, in the absence of acetate, partial induction occurred when Ca²⁺ was present in the medium at concentrations of ≥1 μM. The Ca²⁺ channel-blocking agents Nd³⁺ and nifedipine partially inhibited the external Ca²⁺-supported induction of GSAT mRNA but did not inhibit acetate-supported induction. The calmodulin antagonists trifluoperazine and N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide inhibited both external Ca²⁺-supported and acetate-supported induction. The Ca²⁺ ionophore A23187 caused a transient induction in the dark. These results suggest that Ca²⁺ and calmodulin are involved in the signal transduction pathway linking blue light perception to the induction of GSAT mRNA. The electron transport uncoupler carbonyl cyanide m-chlorophenylhydrazone inhibited acetate-supported induction of GSAT mRNA but did not inhibit external Ca²⁺-supported induction. It is proposed that in the presence of acetate, an internal pool of Ca²⁺ can be mobilized as a second message, whereas in the absence of acetate, internal Ca²⁺ is not available but the requirement for Ca²⁺ can be partially met by an external Ca²⁺ source. The mobilization of internal Ca²⁺ may require energy derived from metabolism of acetate.