Poster: Oxidative Stress

Abs # 146: Differential regulation of High-Light Activated (HLA) genes in Chlamydomonas reinhardtii

Presenter:	Im, Chung-soon , csim@andrew2.stanford.edu
Authors	Im, Chung-soon  (A)   Grossman, Arthur R (A)
Affiliations:	(A): Carnegie Institution of Washington

In previous work, we have used differential display to isolate nine genes that are induced under high light stress in Chlamydomonas reinhardtii. We found that the HLA genes are categorized into two groups according to their response to CO₂ levels as well as high light. All of the genes that appeared to be controlled by both light and CO₂ levels were also shown to be under the control of Cia5, a protein that regulates the responses of Chlamydomonas to low CO₂ conditions. Genes in this category included a carbonic anhydrase, a previously identified low CO₂-induced protein, an ABC transporter of the MRP subfamily, and an unknown protein. The second category of genes specifically activated by high light, even under high CO₂ conditions, encodes a 30 kDa chloroplast membrane protein, a serine hydroxymethyltransferase, a micrococcal nuclease and an ABC transporter. Experiments using DCMU, an inhibitor of photosynthetic electron transport, and mutants devoid of either photosystem I or photosystem II activity show aberrant expression of all of the CO₂/high light-regulated genes, 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 are specifically controlled by high light. We also found that genes specifically activated by high light are also induced by low fluence blue light, but none of the CO₂/high light-regulated genes respond to blue light. The expression of all of HLA genes is abolished in a nph1 mutant generated by RNAi (collaborating w /P. Hegemann), suggesting possible involvement of Nph1 photoreceptor in HLA gene expression. We further investigated the function of light receptors in the high light acclimation process.