Mutations in the Arabidopsis gene immutans cause a variegated phenotype byinactivating a chloroplast terminal oxidase associated with phytoene desaturation

The immutans (im) mutant of Arabidopsis shows a variegated phenotype compri sing albino and green somatic sectors. We have cloned the IM gene by transp oson tagging and show that even stable null alleles give rise to a variegat ed phenotype. The gene product has amino acid similarity to the mitochondri al alternative oxidase. We show that the IM protein is synthesized as a pre cursor polypeptide that is imported into chloroplasts and inserted into the thylakoid membrane. The albino sectors of im plants contain reduced levels of carotenoids and increased levels of the carotenoid precursor phytoene. The data presented here are consistent with a role for the IM protein as a cofactor for carotenoid desaturation. The suggested terminal oxidase functi on of IM appears to be essential to prevent photooxidative damage during ea rly steps of chloroplast formation. We propose a model in which IM function is linked to phytoene desaturation and, possibly, to the respiratory activ ity of the chloroplast.