Ribosomal RNA homologies among distantly related organisms

The similarity in base sequences of ribosomal RNA (rRNA) in widely divergent organisms was compared. Hybrids were prepared between Escherichia coli and pea rRNA on the one hand and the DNA of several organisms on the other. The varying thermal stabilities of these hybrids were taken as evidence for a hierarchy of rRNA relatedness among some higher plants, and for phylogenetic relationships among some species of bacteria, protozoa, fungi, plants, and animals. The large rRNA molecule appears to have evolved faster than the small rRNA molecule.