Sequences complementary to rRNA in rat DNA amount to 0.04% which corresponds to about 300 rRNA genes per haploid genome. 2. Purified DNA-rRNA hybrids had a DNA: rRNA weight ratio of about 1.1:1. Their melting temperature estimated on hydroxyapatite column was 75-80 degrees C. In CsCl gradients hybrids banded at 1.728 g/cm3 as compared to 1.699 g/cm3 for bulk DNA. 3. Length distribution histogram of DNA-rRNA hybrid molecules visualized by electron microscopy revealed that most of them possessed a length of 0.2-0.5 mum which corresponds to 4 X 10(5)-1 X 10(6) daltons, i.e. 670-1600 nucleotide pairs. 4. DNA sequences complementary to rRNA are localized on the heavy shoulder of the main band at a CsCl density of 1.709 g/cm3, and in a DNA fraction which re-associates at a Cot=10(-1)-10(2).
