RIBONUCLEOPROTEIN PARTICLES IN THE AMPHIBIAN OOCYTE NUCLEUS : Possible Intermediates in Ribosome Synthesis

Studies of the sedimentation properties of RNP¹ material from the nucleus of the amphibian oocyte have indicated (1) that there are few, if any, 78S ribosomes in the nucleus, (2) that there are smaller particles sedimenting at 50-55S and 30S, and (3) that the larger of these is the precursor of the 60S subunit of the cytoplasmic ribosomes. Although the nature of the 30S material is not completely clear, it probably includes precursor particles to the 40S ribosomal subunit. Heavy (50-55S) particles are predominant in immature oocytes of Triturus viridescens, whereas in immature oocytes of Triturus and Amblystoma mexicanum they are reduced greatly in amount, but are still detectable. Double-labeling studies of RNA and protein reveal that both types of particle incorporate uridine-³H, but that the 50-55S material of immature oocytes does not incorporate ¹⁴C-labeled amino acids. However, other evidence exists that favors the RNP nature of this material. Sedimentation analyses after SDS extraction show that 50-55S particles contain 40 and 30S RNA, whereas 30S particles contain 20S RNA. These types of RNA represent at least 80% of all the extractable nuclear RNA. The 50-55S particles are probably heterogeneous, including both particles containing mostly 40S RNA and particles containing only 30S RNA.