THE PLANT CELL, Vol 8, Issue 12 2245-2253, Copyright © 1996 by American Society of Plant Biologists
Calcium and Calmodulin Are Involved in Blue Light Induction of the gsa Gene for an Early Chlorophyll Biosynthetic Step in Chlamydomonas
Cs. Im, G. L. Matters and S. I. Beale
Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912
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 [greater or equal
to]1 [mu]M. 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.
