The cox1 gene from Euglena gracilis: a protist mitochondrial gene without introns and genetic code modifications.

We present the nucleotide sequence of the cox 1 gene encoding subunit 1 of cytochrome c oxidase in Euglena gracilis, the first report on a mitochondrial gene from this protist. Its study reveals that the Euglena mitochondrial genome does not appear as a compact and homogeneous structure and that its A+T content is high (about 76%) whereas this value is less than 50% in nuclear DNA. The Euglena cox1 gene does not exhibit any intron, and an amino-acid alignment of Euglena COX1 with homologous proteins shows that the universal genetic code is used. Comparisons of the genomic and cDNA sequences of Euglena cox1 indicate that the transcript does not undergo RNA editing as found in trypanosomes and in higher plants. The phylogeny obtained with COX1 protein sequences is in agreement with that obtained with nuclear rRNA sequences and places Euglena and Trypanosoma far apart from other eukaryotes. This result strengthens the hypothesis that these protists represent the earliest mitochondrion-containing organisms.