RNA synthesis in fetal rat liver

The synthesis of RNA in the livers of 20-day-old fetal rats has been studied by the incorporation of ³²P_i and differential extraction of RNA with 0.14 m NaCl and phenol at 3, 45, and 80 °. Substantial incorporation of isotope into RNA occurred over a 4-hour period after the injection of ³²P_i into the mother. Four fractions of RNA were obtained, each with widely differing properties. The RNA extracted at 3 ° was subfractionated on the basis of solubility in 1 m NaCl, and the resulting fractions were shown by various criteria to consist of ribosomal and transfer RNA′s. The kinetics of labeling were analogous to those previously observed in adult livers. There was no evidence of turnover of the -pCpCpA terminus, but the transfer RNA of fetal liver had the same capacity for accepting leucine and phenylalanine as adult liver transfer RNA.The RNA obtained at 45 ° incorporated a great deal of ³²P_i 15 minutes after injection, but thereafter the incorporation relative to the total continually declined. Peaks of [³²P]-RNA sedimented at 45 and 35S at early intervals, but by 4 hours these were largely obscured by new peaks which appeared at 28 and 18S. Because of these characteristics and a similarity in base composition, this RNA is considered to contain precursors of ribosomal RNA.The RNA extracted at 80 ° combined the highest specific radioactivity with the least amount of RNA. The isotopic and chemical base compositions were both DNA-like, and in density gradient centrifugations the labeled RNA was polydisperse. The possibility is considered that this RNA represents messenger RNA.A small amount of very heavily labeled RNA was obtained from the tissue residue after the usual treatment by heating at 100 ° with 10% NaCl. The chemical base composition was ribosomal-like, but isotopically its composition was mixed. The nature of this RNA is discussed.