Identification and regulation of high light-induced genes in Chlamydomonas reinhardtii

We have used restriction fragment differential display for isolating genes of the unicellular green alga Chlamydomonas reinhardtii that exhibit elevated expression on exposure of cells to high light. Some of the high light-activated genes were also controlled by CO2 concentration. Genes requiring both elevated light and low CO2 levels for activation encoded both novel polypeptides and those that function in concentrating inorganic carbon (extracellular carbonic anhydrase, low CO2-induced protein, ABC transporter of the MRP subfamily). All the genes in this category were shown to be under the control of Cia5, a protein that regulates the responses of C. reinhardtii to low-CO2 conditions. Genes specifically activated by high light, even under high-CO2 conditions, encoded a 30 kDa chloroplast membrane protein, a serine hydroxymethyltransferase, a nuclease, and two proteins of unknown function. Experiments using DCMU, an inhibitor of photosynthetic electron transport, and mutants devoid of either photosystem I or photosystem II activity, showed aberrant expression of all the genes regulated by both CO2 and high light, suggesting that redox plays a role in controlling their expression. In contrast, there was little effect of DCMU or lesions that block photosynthetic electron transport on the activity of genes that were specifically controlled by high light.