Algal photoprotection is regulated by the E3 ligase CUL4-DDB1.

Light is essential for photosynthesis, but the amounts of light that exceed an organism's assimilation capacity can cause serious damage. Photosynthetic organisms minimize such potential harm through protection mechanisms collectively referred to as non-photochemical quenching. One mechanism of non-photochemical quenching called energy-dependent quenching (qE quenching) is readily activated under high-light conditions and dissipates excess energy as heat. LIGHT-HARVESTING COMPLEX STRESS-RELATED PROTEINS 1 and 3 (LHCSR1 and LHCSR3) have been proposed to mediate qE quenching in the green alga Chlamydomonas reinhardtii when grown under high-light conditions. LHCSR3 induction requires a blue-light photoreceptor, PHOTOTROPIN (PHOT), although the signal transduction pathway between PHOT and LHCSR3 is not yet clear. Here, we identify two phot suppressor loci involved in qE quenching: de-etiolated 1 (det1) and damaged DNA-binding 1 (ddb1). Using a yeast two-hybrid analysis and an inhibitor assay, we determined that these two genetic elements are part of a protein complex containing CULLIN 4 (CUL4). These findings suggest a photoprotective role for the putative E3 ubiquitin ligase CUL4-DDB1 in unicellular photosynthetic organisms that may mediate blue-light signals to LHCSR1 and LHCSR3 gene expression.