HYBRIDIZATION OF RAPIDLY LABELED NUCLEAR RIBONUCLEIC ACIDS.

RNA, prepared from strain L fibroblasts which were incubated for 30 minutes in radioactive cytidine, was examined by quantitative DNA-RNA hybridization tests, autoradiography, and sedimentation analysis. About 60 percent of the radioactive RNA consists of 38 to 45S molecules located in the nucleoli and capable, under certain circumstances, of forming hybrids with DNA. Unlabeled 28S and 18S ribosomal RNA can compete with these components for DNA sites, indicating that the conversion of nucleolar RNA to ribosomal RNA occurs in such a way that base sequences are preserved. The polydisperse components associated with the chromatin region contain base sequences different from ribosomal and 4S RNA, and consequently have a uniquely high specific activity. These components are the only detectable hybridizing species in unfractionated preparations of RNA.