RNA homologies between protozoa

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.

Nucleic acid homologies among species of Saccharomyces

Evolutionary divergence among species of the yeast genus Saccharomyces was estimated from measurements of deoxyribonucleic acid (DNA)/DNA and ribosomal ribonucleic acid (RNA)/DNA homology. Much diversity was found in the DNA base sequences with several species showing little or no homology to the three reference species, S. cerevisiae, S. lactis, and S. fragilis. These three reference species also showed little or no homology to each other. On the other hand the diversity among ribosomal RNA base sequences was small since most species showed a high degree of homology to the reference species. The arrangement of species based on ribosomal RNA homologies agrees in most cases with current taxonomic groupings. A yeast hybrid (S. fragilis x S. lactis) was shown to contain two nonhomologous genomes. A minimum genome size of 9.2 x 10(9) daltons for S. cerevisiae was calculated from the rate of DNA renaturation.