Hybridizable ribonucleic acid of rat brain.

1. Cerebral RNA of adult and newborn rats was labelled in vivo by intracervical injection of [5-(3)H]uridine or [(32)P]phosphate. Hepatic RNA of similar animals was labelled by intraperitoneal administration of [6-(14)C]orotic acid. Nuclear and cytoplasmic fractions were isolated and purified by procedures involving extraction with phenol and repeated precipitation with ethanol. 2. The fraction of pulse-labelled RNA from cerebral nuclei that hybridized to homologous DNA exhibited a wide range of turnover values and was heterogeneous in sucrose density gradients. 3. Base composition of the hybridizable RNA was similar to that of the total pulse-labelled material; both were DNA-like. 4. Pulse-labelled cerebral nuclear RNA hybridized to a greater extent than cytoplasmic RNA for at least a week after administration of labelled precursor. This finding suggested that cerebral nuclei contained a hybridizable component that was not transferred to cytoplasm. 5. The rates of decay of the hybridizable fractions of cerebral nuclei and cytoplasm were faster in the newborn animal than in the adult. Presumably a larger proportion of labile messenger RNA molecules was present in the immature brain. 6. Cerebral nuclear and cytoplasmic RNA fractions from newborn or adult rats, labelled either in vivo for periods varying from 4min. to 7 days or in vitro by exposure to [(3)H]-dimethyl sulphate, uniformly hybridized more effectively than the corresponding hepatic preparation. These data suggested that a larger proportion of RNA synthesis was oriented towards messenger RNA formation in brain than in liver.