Immunological identity of proteins that bind stored 5S RNA in Xenopus oocytes

In small oocytes of Xenopus laevis, the three most abundant proteins are isolated as basic polypeptides with molecular weights of 48 kD (P48), 43 kD (P43) and 40 kD (P40, also known as transcription factor IIIA). All three proteins share common properties in being able to bind specifically ribosomal 5S RNA molecules and influence, in different ways, their rates of production and utilization. It has been shown by biochemical analysis and immunological characterization that the three proteins are structurally distinct and are most probably the products of different genes. Immunostaining and radio-immunoassays indicate that both P48 and P43 have diverged considerably in structure between the amphibian genera Xenopus and Triturus. Antibodies raised against the transcription factor for Xenopus laevis 5S RNA genes (P40/TFIIIA) do not cross-react with the transcription factor isolated from oocytes of the closely related species Xenopus borealis. A protein equivalent of TFIIIA is not found in 5S RNA-containing RNP storage particles of Triturus oocytes. The functions of the three Xenopus oocyte proteins in transporting 5S RNA between different cellular compartments are considered in the light of these variations.