LIGHT INDUCES ACCUMULATION OF ISOCITRATE LYASE MESSENGER-RNA IN A CAROTENOID-DEFICIENT MUTANT OF CHLAMYDOMONAS-REINHARDTII

A cDNA with sequence similarity to isocitrate lyase (ICL) genes was is olated from the unicellular eukaryotic green alga Chlamydomonas reinha rdtii as a light-induced mRNA in the carotenoid biosynthetic mutant st rain FN68. The 416 amino acid open reading frame shows significant seq uence similarity to isocitrate lyases of bacteria (70%), molds (48%), yeasts (45%), and plants (47%). Expression of the Chlamydomonas ICL ge ne was tested in the mutant strain FN68, which when grown in the dark fails to accumulate carotenoids and is deficient in chlorophyll, and i n CC400G, a strain that accumulates wild-type levels of carotenoids an d chlorophyll. In vegetative CC400G cells, ICL mRNA accumulated to a h igh level in the dark and declined to a barely detectable level within 30 min of exposure to light. This response was more sensitive to whit e (tungsten filament) or red light than green or blue light, excluding cryptochrome and rhodopsin as the photoreceptor These results are con sistent with excitation by chlorophyll and/or a phytochrome-related ph otoreceptor. In vegetative FN68 cells, ICL mRNA abundance was very low in the dark, but increased dramatically in response to light. At inte nsities above threshold, excitation by far-red or red light-induced IC L mRNA accumulation to the highest levels. The threshold of the respon se was lowest for far-red and blue light. These results are consistent with excitation of a photochromic far-red-responsive pigment.