Characterization of Protein Bound to Rapidly‐Labelled RNA in Polyribosomes from Rat Liver

A complex of rapidly labelled RNA and protein that can be liberated from polysomes by treatment with EDTA has been characterized in attempts to establish whether it is derived from the nuclear particles (“informofers”) containing RNA of DNA‐like nucleotide composition. After short time of labelling with [ 3 H]orotic acid, polysomes were isolated, either by a method involving Triton X‐100, or without any detergent (“free” polysomes). After treatment of polysomes with EDTA, the sedimentation rate of the rapidly‐labelled RNA was considerably higher than that expected for pure RNA. CsCl density gradient centrifugation demonstrated that the buoyant density of the labelled material had a value intermediate between that of pure RNA and protein. Treatment with sodium deoxycholate or with 0.5 M KCl, increased considerably the buoyant density of the labelled material, while that of the ribosomal sub‐units was only slightly affected. The results indicate that in polysomes the rapidly labelled RNA exists in a complex with protein that can be selectively removed by sodium deoxycholate or KCl. The proteins released by treatment with deoxycholate or with KCl were characterized by polyacrylamide gel electrophoresis. One of these proteins behaved similarly to the protein‐component of the nuclear particle containing RNA of DNA‐like base composition. It is suggested that this protein follows the rapidly labelled RNA (presumably messenger RNA) during its transfer from the nucleus to the polysomes.