The carotenoid-deficient mutant, C-6E, of Scenedesmus obliquus is blocked at the site of phytoene synthase

In contrast to the wild type strain of Scenedesmus , mutant C-6E synthesized only trace amounts of the carotenoids violaxanthin and lutein during prolonged heterotrophic growth. All other carotenoids and carotenoid precursors, such as phytoene, were undetectable. Additionally, only reduced levels of chlorophyll a and no chlorophyll b were formed. To evaluate the potential site of inhibition in the pathway for carotenoid biosynthesis the enzymatic activities of geranylgeranyl pyrophosphate synthase and phytoene synthase were assayed in cell-free extracts. Both enzymes were highly active in extracts of the wild type but only geranylgeranyl pyrophosphate synthase was active in comparable extracts from mutant C-6E . This observation strongly indicates that the phenotype of C-6E results from either a mutation of the phytoene synthase structural gene or of a regulatory gene involved in expression of this enzyme. Other phenotypic effects on composition and structure of the photosynthetic apparatus are discussed as a secondary consequence of the carotenoid deficiency in the thylakoid membranes.