NUCLEOTIDE-SEQUENCE OF THE COX3 GENE FROM CHONDRUS-CRISPUS - EVIDENCETHAT UGA ENCODES TRYPTOPHAN AND EVOLUTIONARY IMPLICATIONS

We present the nucleotide sequence of the gene encoding subunit 3 of c ytochrome c oxidase in Chondrus crispus, the first report on a mitocho ndrial gene from a red alga. Amino acid alignment with homologous prot eins shows that tryptophan is specified by UGA, as in the mitochondria l code of most organisms other than green plants. However, phylogeneti c analyses of cox3 amino acid and nucleotide sequences indicate that C .crispus COX3 is related to the green-plant mitochondrial lineage. No RNA editing was detected on the corresponding transcript. As the only known photosynthetic eukaryotes than both share an immediate mitochond rial ancestor with green plants and exhibit features characteristic of non-plant mitochondria, ie, a small-sized mitochondrial genome and a modified genetic code, rhodophytes may be thought of as an intermediat e evolutionary link at the root of the green-plant mitochondrial linea ge.