Studies on ribosomal RNA synthesis in vivo in rat liver during short-term protein malnutrition

To better understand the increased incorporation of [14C] orotate into rRNA, as detailed study of liver rRNA synthesis was performed on young rats fed a 6% casein diet for 1 week. From 10 to 180 minutes after i.v. injection of [14C] orotate, nucleolar RNA specific activity of malnourished rats was 1.6 times higher than controls. Polyacrylamide gel electrophoresis revealed that 45S pre-rRNA from malnourished rats was synthesized more rapidly than controls. Specific activities of other nucleolar RNA species was increased but the time of appearance of labeled peaks did not change. The cellular cytoplasmic rRNA pool was reduced to 70% of controls, the time of appearance of labeled rRNA in the cytoplasm was the same, but cytoplasmic rRNA specific activity was 2.1 times greater than controls. Absorbance profiles and radioactivity patterns of nucleoplasmic RNA were similar in the two groups of rats. Nucleoplasmic radioactivity patterns were complex and heterodispersed, and the amount of radioactivity was greatest in RNA from malnourished rat liver. Our data indicate that short-term dietary protein restriction leads to cellular events which increase the synthesis of nuclear RNA. Nucleolar RNA is processed and transported to the cytoplasm in a manner similar to controls.