PLANT PHYSIOLOGY , Vol 109, Issue 2 471-479, Copyright © 1995 by American Society of Plant Biologists
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GENE REGULATION AND MOLECULAR GENETICS |
Blue-Light-Regulated Expression of Genes for Two Early Steps of Chlorophyll Biosynthesis in Chlamydomonas reinhardtii
G. L. Matters and S. I. Beale
Division of Biology and Medicine, Brown University, Providence, Rhode Island 02912
In light:dark-synchronized cultures of Chlamydomonas reinhardtii, the genes
encoding the enzymes for two early steps of chlorophyll biosynthesis,
glutamate-1-semialdehyde aminotransferase (gsa) and [delta]-aminolevulinic
acid dehydratase (alad), are expressed at high levels early in the light
phase, just prior to a rapid burst of chlorophyll synthesis. Induction of
gsa mRNA in synchronized cells is totally dependent on light, whereas
induction of alad mRNA occurs to approximately one-half the light-induced
level even in cells kept in the dark during the light phase and appears to
be dependent on the cell cycle or a circadian rhythm. gsa mRNA and alad
mRNA accumulation is induced by light that was passed through blue (400-480
nm) or green (490-590 nm) filters but not by light that was passed through
orange (>560 nm) or red (>610 nm) filters, indicating the
participation of a blue-light photoreceptor system rather than a
protochlorophyllide- or rhodopsin-based photoreceptor. Light induction of
gsa mRNA accumulation is absent in a carotenoid-deficient mutant, which
suggests that a carotenoid-containing blue-light photoreceptor is involved.
In contrast, pretreatment of wild-type cells with either of two flavin
antagonists, phenylacetic acid and KI, does not prevent the light
induction. In the later part of the light phase, the gsa mRNA level
decreases more rapidly than that of alad mRNA. Turnover studies indicate
that the half-life of alad mRNA is twice that of gsa mRNA. This difference
in mRNA stability partially accounts for the more rapid decline in gsa mRNA
levels after the peak of light induction is reached. Thus, differential
blue-light induction and stability of mRNAs regulates the expression of
these two chlorophyll biosynthetic genes.
