The metabolism of high-molecular-weight RNA in the nuclear and cytoplasmic fractions of newborn and adult rat brain was investigated after the intracranial administration of [32P]Pi. In young brain, a considerable proportion of the newly synthesized radioactive RNA is transferred to the cytoplasm, in contrast with the adult brain, where there appears to be a high intranuclear turnover. Electrophoretic analysis of the newly synthesized RNA showed that processing of the rRNA precursor to yield the 28S and 18S rRNA may be more rapid in the adult than in the young, although most of the adult rRNA in the nucleus is not transferred to the cytoplasm. In young brain, processing is probably tightly coupled to transport of rRNA into the cytoplasm, so that 28S and 18S rRNA are not subjected to possible degradation within the nucleus. Polyadenylated RNA turns over in concert with high-molecular-weight RNA in the nuclei of the adult rat brain. In the cytoplasm the polyadenylated RNA has a higher turnover rate relative to rRNA. In the young brain the polyadenylated RNA is transferred to the cytoplasm along with rRNA, although polyadenylated RNA is transported into the cytoplasm at a faster rate. The nuclear and cytoplasmic polyadenylated RNA species of young brain are larger than their corresponding adult counterparts. These results suggest that there are considerable changes in the regulation of the nucleo-cytoplasmic relationship of rRNA and polyadenylated RNA during the transition of the brain from a developing replicative phase to an adult differentiated and non-dividing state.

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