Abstract
Abstract A rapidly labeled 45 S ribonucleoprotein complex has been isolated and characterized from purified rat liver nuclei. The chemical and physical characterization of the nuclear RNA-protein complex revealed the following properties. (a) An RNA component that was identified as messenger RNA on the basis of rapid incorporation of RNA precursors, molecular wieght, nucleotide base composition, and RNA-DNA hybridization. (b) A protein component, distinct from ribosomal proteins or histones with respect to its electrophoretic properties. (c) An RNA-protein complex different from ribosomal subunits with respect to its structural organization and its nucleic acid to protein ratio. These studies suggest that the 45 S ribonucleoprotein complex does not represent a precursor to cytoplasmic ribosomal subunits but represents a complex of newly synthesized mRNA and specific nuclear proteins. The formation of such a specific mRNA-protein complex within the nucleus could provide a mechanism for the release of mRNA from its DNA template, or the transport of mRNA from the nucleus to the cytoplasm, or both.
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