Cbr, an algal homolog of plant early light-induced proteins, is a putative zeaxanthin binding protein

The cbr gene, previously cloned from the unicellular green alga Dunaliella bardawil, is transcriptionally and translationally activated in parallel to accelerated carotenogenesis in response to light stress conditions. The product of cbr, structurally similar to Elips (early light-induced proteins of higher plants), is associated with a minor light harvesting complexes of photosystem II component (Levy, H., Gokhman, I., and Zamir, A. (1992) J. Biol. Chem. 267, 18831-18836). This study examines the relationship between the induction of Cbr and another plant response to light stress, the deepoxidation of violaxanthin to zeaxanthin. A parallel between the two processes was observed in cells exposed to high light, starved for sulfate, or treated with norflurazon, a herbicide inducing photooxidative damage by inhibiting de novo carotenoid biosynthesis. When highly illuminated cells were returned to normal light, Cbr decayed in parallel to the reepoxidation of zeaxanthin to violaxanthin. Evidence for the physical association of Cbr and zeaxanthin was provided by nondenaturing gel electrophoresis. In cells transferred from low to high light, zeaxanthin was associated with the faster migrating of two electrophoretically resolved fractions of light harvesting complexes of photosystem II that also contained Cbr. In cells growing under normal light, violaxanthin was bound equally to the two fractions. Based on these results we propose that Cbr/early light-induced proteins bind zeaxanthin to form photoprotective complexes within the light-harvesting antennae.