Characterization of a nucleolar residue fraction that specifically transcribes preribosomal RNA

Treatment of isolated nucleoli with Sarkosyl (2%) dissociated 99.5% of the proteins. The residual DNA-protein complex contained the endogenous transcriptional activity which had a high fidelity of RNA synthesis. Electron microscopic analysis of this residue fraction showed the presence of 150-200A diameter protein globules present along the length of some of the DNA fibers. SDS-polyacrylamide gel electrophoretic analysis of the proteins of the complex indicated that the subunits of purified RNA polymerase I were only a minor component of this complex. Associated with the complex were the U3 and 5S RNA.