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 earl y chlorophyll biosynthetic enzyme glutamate 1-semialdehyde aminotransf erase (GSAT), is specifically induced by blue light in cells synchroni zed in a 12-hr-light and 12-hr-dark regime, Light induction required t he presence of a nitrogen source in the incubation medium, Maximal ind uction also required acetate, However, in the absence of acetate, part ial induction occurred when Ca2+ was present in the medium at concentr ations of greater than or equal to 1 mu M. The Ca2+ channel-blocking a gents Nd3+ and nifedipine partially inhibited the external Ca2+-suppor ted induction of GSAT mRNA but did not inhibit acetate-supported induc tion. The calmodulin antagonists trifluoperazine and N-(6-aminohexyl)- 5-chloro-1-naphthalenesulfonamide inhibited both external Ca2+-support ed 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 blu e light perception to the induction of GSAT mRNA, The electron transpo rt uncoupler carbonyl cyanide m-chlorophenylhydrazone inhibited acetat e-supported induction of GSAT mRNA but did not inhibit external Ca2+-s upported induction. It is proposed that in the presence of acetate, an internal pool of Ca2+ can be mobilized as a second message, whereas i n the absence of acetate, internal Ca2+ is not available but the requi rement for Ca2+ can be partially met by an external Ca2+ source, The m obilization of internal Ca2+ may require energy derived from metabolis m of acetate.