Isolation and DNA‐RNA Hybridization Properties of Small‐Molecular‐Weight Nuclear RNA Components from Baby‐Hamster‐Kidney Cells

Several small‐molecular‐weight nuclear RNA components (snRNAs) have been isolated and purified from nuclei of baby hamster kidney cells. The proportion of the cell genome complementary to the different snRNA components was investigated by DNA‐RNA hybridization. The values for saturation range between 0.4 × 10−4% and 0.8 × 10−4% which correspond to about 2000 genes for each of the snRNA components per cell. For comparison the saturation value for rRNA is 2.2 × 10−4% which corresponds to about 200 genes per cell. Saturation experiments with mixtures of the snRNA components indicate that there are separate genes for the different snRNA components. Hybridization competition experiments show that no base sequence homologies exist between the snRNAs and other RNAs such as rRNA, 5‐S RNA and tRNA. The DNA complementary to the snRNAs is distributed throughout the band of the bulk DNA of buoyant‐density fractionated DNA. This is in contrast to the DNA complementary to rRNA which bands as a heavy satellite. Hence the snRNA components can not be fragments of the non‐conserved parts of the rRNA precursor.