In Vitro Incorporation of Labeled Amino Acids into Nuclei Isolated from Rat Liver. Nuclear fractions isolated from rat liver are able to incorporate labeled amino acids in vitro .
Evidence is given that the incorporation mainly is into nuclear proteins.
By fractionation of the protein after incubation of the intact preparations with the labeled amino acid, the residue not extractable in M NaCl was shown to be the most active fraction. The proteins of the DNA containing fraction were less labeled. The experiments strongly suggest that active RNP-particles are present in rat liver nuclei. No conclusive evidence could be obtained in support of the hypothesis that a second site for amino acid incorporation into proteins is present in the nuclei.