Plant Molecular Biology 33: 381–392, 1997.
1997 Kluwer Academic Publishers. Printed in Belgium.
Light induces accumulation of isocitrate lyase mRNA in a
carotenoid-deﬁcient mutant of Chlamydomonas reinhardtii
, Kenneth Foster
and Karen Kindle
Department of Physics, Syracuse University, Syracuse, NY 13244, USA;
Section of Biochemistry, Molecular and
Cell Biology, Cornell University, Ithaca, NY 14853, USA;
Current address: Mt. Sinai Medical Center,
Department of Ophthalmology, Box 1183, New York, NY 10029-6574, USA;
Current address: Plant Science
Center, Biotechnology Building, Cornell University, Ithaca, NY 14853, USA (
author for correspondence)
Received 29 April 1996; accepted in revised form 29 August 1996
Key words: Chlamydomonas reinhardtii, isocitrate lyase, chlorophyll, photomorphogenesis, photooxidation,
A cDNA with sequence similarity to isocitrate lyase (ICL) genes was isolated from the unicellular eukaryotic green
alga Chlamydomonas reinhardtii as a light-induced mRNA in the carotenoid biosynthetic mutant strain FN68. The
416 amino acid open reading frame shows signiﬁcant sequence similarity to isocitrate lyases of bacteria (70%),
molds (48%), yeasts (45%), and plants (47%).
Expression of the Chlamydomonas ICL gene was tested in the mutant strain FN68, which when grown in the
dark fails to accumulate carotenoids and is deﬁcient in chlorophyll, and in CC400G, a strain that accumulates
wild-type levels of carotenoids and chlorophyll. In vegetative CC400G cells, ICL mRNA accumulated to a high
level in the dark and declined to a barely detectable level within 30 min of exposure to light. This response was
more sensitive to white (tungsten ﬁlament) or red light than green or blue light, excluding cryptochrome and
rhodopsin as the photoreceptor. These results are consistent with excitation by chlorophyll and/or a phytochrome-
related photoreceptor. In vegetative FN68 cells, ICL mRNA abundance was very low in the dark, but increased
dramatically in response to light. At intensities above threshold, excitation by far-red or red light-induced ICL
mRNA accumulation to the highest levels. The threshold of the response was lowest for far-red and blue light.
These results are consistent with excitation of a photochromic far-red-responsive pigment.
onacetateasasolecarbonsource.In oilseedplants, the
glyoxylate cycle is used during seedling development
todegradefats foruseas an energysourceby the grow-
ing seedling. In most eukaryotic organisms, isocitrate
lyase (ICL) is localized to the glyoxysome, a special-
ized form of peroxisome. The gene encoding ICL has
been isolated from a number of embryophyteswhere it
has been shown to be regulated both by developmental
cues and in response to light. It is actively expressed
in late embryogeny and in germinating seeds, and its
activity is repressed as the photosynthetic capacity of
the plant increases and glyoxysomes are replaced by
peroxisomes [10, 13, 67]. ICL enzyme activity has
also been noted in senescing or detached organs [43,
51] and in pollen .
We have isolated a Chlamydomonas reinhardtii
cDNA that encodes a protein that is closely related
to isocitrate lyases; it is induced in response to light in
the carotenoid biosynthetic mutant FN68. In the stud-
ies described below, we show that in C. reinhardtii,the
accumulation of ICL mRNA is regulated differently in
dark-grown vegetative cells of the carotenoid mutant
FN68 and in vegetative cells of CC400G, which is
wild-type with respect to carotenoid and chlorophyll
accumulation and has a high photosynthetic capacity.
When CC400G cells are grown under a 12 h light/12 h