Ochromonas mitochondria contain a specific chloroplast protein.

Antibody raised against the small subunit of ribulose-1,5-bisphosphate carboxylase [3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39] of Chlamydomonas reinhardtii labeled the mitochondria as well as the chloroplast of the chrysophyte alga Ochromonas danica in sections prepared for immunoelectron microscopy by the protein A-gold technique. The same antibody labeled the chloroplast but not the mitochondria of C. reinhardtii. A quantitative study of labeling in dark-grown, greening (32 hr light), and mature green cells of O. danica revealed that anti-small-subunit staining in the mitochondria increased progressively in the light as it does in the plastid. Antibody to the large subunit of the enzyme did not label the mitochondria of either O. danica or C. reinhardtii. In view of the recent demonstrations of homologous DNA sequences in the mitochondrial and chloroplast genomes of higher plants, we suggest that the DNA sequence coding for the small subunit has migrated to the mitochondria from nucleus or chloroplast and is expressed within the organelle.