| Abstract: | The pre-rRNA and rRNA components of rat and mouse liver nucleolar RNA were analysed. It was shown that upon denaturation, part of the 32 S pre-rRNA is converted into 28 S rRNA and 12 S RNA. The 12 S RNA from mouse (Mr, 0.36 X 10(6)) is larger than the one from rat (Mr, 0.32 X 10(6). The 12 S RNA chain is intact and resists denaturation treatment. The non-covalent binding of this RNA with nucleolar 28 S rRNA is stronger than that of 5.8 S rRNA with 28 S rRNA. Hybridization with a rat internal-transcribed spacer rDNA fragment identifies 12 S RNA as corresponding to the 5'-end non-conserved segment of 32 S pre-rRNA, including 5.8 S rRNA. The significance of the formation of a 12 S precursor to 5.8 S rRNA in the biogenesis of ribosomes in mammalian cells is discussed. |
