Incorporation of labelled precursors into the electrophoretic fractions of rat-liver ribonucleic acid.

1. Incorporation of [(32)P]orthophosphate and of [2-(14)C]orotic acid into rat-liver RNA was studied by agar-gel electrophoresis by using u.v.-densitometry and radioautography of dried agar electrophoretograms. 2. During the electrophoresis some low-molecular-weight contaminants, including inorganic phosphate present in the RNA preparations, were separated from the RNA fractions. Since nucleoside mono-, di- and tri-phosphates still interfered, the RNA preparations had to be subjected to a purification procedure [Sephadex G-25 or Dowex 1 (X8)]. 3. In RNA extracted from cytoplasm, isolated microsomes or ribosomes, whatever variations were made in the phenol procedure no special rapidly labelled RNA fraction was detected other than ;soluble' RNA and the ribosomal RNA components. 4. When the whole homogenate or cytoplasmic fraction was treated only with phenol (pH6) a considerable part of the cytoplasmic RNA was not extracted. The treatment of the cytoplasmic fraction with sodium dodecyl sulphate before the addition of phenol increased the yield of the high-molecular-weight RNA and at the same time a higher specific activity was found for the faster ribosomal RNA component. 5. The presence of four distinct rapidly labelled RNA fractions was established in the RNA not extracted by phenol, and they moved slower than the ribosomal RNA. They were extracted only with the use of phenol-sodium dodecyl sulphate at an elevated temperature.