Algal photoprotection is regulated by the E3 ligase CUL4–DDB1 DET1

Light is essential for photosynthesis, but the amounts of light that exceed an organism’s assimilation capacity can cause serious damage 1 . Photosynthetic organisms minimize such potential harm through protection mechanisms collectively referred to as non-photochemical quenching 2 . 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 3 . LHCSR3 induction requires a blue-light photoreceptor, PHOTOTROPIN (PHOT) 4 , 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 ) 5 and damaged DNA-binding 1 ( ddb1 ) 6 . 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 DET1 in unicellular photosynthetic organisms that may mediate blue-light signals to LHCSR1 and LHCSR3 gene expression.