Suppression of the light sensitivity of a zeaxanthin- and lutein-deficient mutant of Chalmydomonas reinhardtii

Abstract

Xanthophylls are essential in protecting the chloroplast from photooxidative damage. The zeaxanthin- and lutein-deficient double mutant of Chlamydomonas reinhardtii, npq1 lor1, shows negligible energy-dependent nonphotochemical quenching of chlorophyll fluorescence and undergoes irreversible photooxidative bleaching at moderate light intensities. When transferred from low to moderate light, the cellular chlorophyll content of the double mutant decreases to undetectable levels, in parallel with an increase in lipid peroxides and the disappearance of thylakoid membranes. We have isolated extragenic suppressors of the npq1 lor1 light sensitivity. Approximately half of the suppressors show the same pigment composition as the parental strain, thereby identifying photoprotective components not directly related to xanthophyll metabolism. Several alleles of the NPQ2 gene, which affects zeaxanthin epoxidase activity, were identified during the suppressor screen. The different npq1 lor1 npq2 strains show varying degrees of constitutive accumulation of zeaxanthin, from zeaxanthin as the only xanthophyll to a ratio of zeaxanthin+ antheraxanthin : violaxanthin of 1:1. When grown in low light, npq1 lor1 npq2 mutants have a light-harvesting complex protein composition similar to that of the wild type, but the major proteins are absent when the mutants are grown in high light. All the strains survive in high light. The null and partial-loss-of-function alleles that we studied are dominant suppressors of the npq1 lor1 light sensitivity. This suggests that minor amounts of zeaxanthin in the chloroplast are sufficient for protection against photooxidative damage.