Regulation of carotenoid biosynthesis genes in response to light in Chlamydomonas reinhardtii.

Carotenoids are ubiquitous and essential components of photosynthetic tissues in plants, algae and cyanobacteria. They participate in the light harvesting process and prevent photooxidative damage of the photosynthetic apparatus. Although de-etiolation and growth under different light conditions were reported to have pronounced effects on carotenoid contents in higher plants and algae, very little is known about the light regulation of carotenogenesis on a molecular level. In the present study, we chose the unicellular green alga Chlamydomonas reinhardtii to investigate the regulation of carotenoid biosynthesis genes in response to light. The carotenoid genes phytoene synthase and phytoene desaturase were selected for gene expression studies. Both phytoene synthase and phytoene desaturase revealed a fast up-regulation in response to light, which seemed to be due to transcriptional control. Only blue light was effective whereas illumination with red light did not lead to elevated transcript levels of phytoene synthase and phytoene desaturase. The inhibition of photosynthesis did not abolish the light induction of carotenoid genes. Comparison with published results showed that the carotenoid genes are simultaneously expressed with other genes involved in chlorophyll biosynthesis and light harvesting. This simultaneous expression may represent one mechanism for the coordinated biosynthesis of carotenoids, chlorophylls and the proteins of the photosynthetic apparatus.