MITOCHONDRIAL DNA .4. INTERACTION OF RIBOPOLYNUCLEOTIDES WITH COMPLEMENTARY STRANDS OF CHICK-LIVER MITOCHONDRIAL DNA

1. 1. The complementary strands of chick-liver mitochondrial DNA have been separated in a preparative alkaline-CsCl gradient. The equilibrium densities of l-strand DNA were 1.746 and 1.716, and of H-strand DNA 1.788 and 1.732 g/cm3 in alkaline and neutral CsCl, respectively (Escherichia coli DNA, 1.772 and 1.710 g/cm3). 2. 2. In the presence of a molar excess of poly (I, G) the density of the H-strand is 1.734 g/cm3 and that of the L-strand > 1.79. In the presence of a molar excess of poly U the density of the H-strand is 1.736 and that of the L-strand > 1.77. 3. 3. If H- and L strand DNA are present in equal amounts in the same neutral-CsCl gradient, only one aggregate band is obtained at about 1.726 g/cm3 (native density 1.708 g/cm3). Complete aggregation is also observed between rat mitochondrial L-strand DNA (density 1.707 g/cm3) and chick H-strand DNA, but not between chick H-strand DNA and denatured DNA of chick nuclei, bacteriophage T4 or Micrococcus lysodeikticus. 4. 4. Aggregation is prevented by denaturing mitochondrial DNA in the presence of a molar excess of poly (I, G) but not by an excess of poly U. Denatured mitochondrial DNA, aggregated in CsCl, binds much less polyribonucleotide than the L-strand it contains. It is, possible therefore, that aggregation involves the sites in the DNA that bind polyribonucleotides. 5. 5. We conclude that the L-strand of chick mitochondrial DNA is rich in dA and dC, and in dC-rich clusters, whereas the H-strand is rich in dT-rich clusters. Since the H-strand acts as the messenger strand in rat liver it is possible that dT-rich clusters are involved in the transcription of mitochondrial DNA. © 1969.