RIBOSOMAL RNA SYNTHESIS IN GERMINATING BLACK EYE PEA (VIGNA UNGUICULATA) .2. SYNTHESIS AND MATURATION OF RIBOSOMES IN LATER STAGES OF GERMINATION

1. Cycloheximide and actinomycin D both inhibit protein synthesis in the axes of 62-h germinated plantlets. Actinomycin D does not inhibit the amino acid incorporation in the cotyledons which normally occurs at a lower rate than in the axes. 2. 32P-labeling experiments show that (a) the synthesis of complete ribosomes ceases sometime between 30 and 40 h after germination and (b) that this synthesis has still not resumed by 72 h. 3. During these stages, the newly transcribed rRNA is associated with cytoplasmic particles which resemble the heavier and the lighter ribosomal subunits with respect to their RNA components, but which sediment significantly more slowly on sucrose gradients than do the normal subunits. 4. On sucrose density gradients the radioactivity profile of RNA isolated from the ribosomes reveals two peaks which show a slightly lower density than the normal 28-S and 18-S rRNA. 5. Both 5-S RNA and tRNA are synthesized during these stages. The newly synthesized 5-S RNA is exclusively located in the heavier subunit but neither the heavier nor the lighter subunit contains any radioactive tRNA. 6. The particles containing the newly synthesized RNA are slowly transformed into mature monomeric ribosomes, apparently without undergoing prior degradation, indicating that these are probably the precursor to normal ribosomes. 7. Actinomycin D does not prevent the transformation of the precursor particles into mature ribosomal subunits but it inhibits their conversion into monomeric ribosomes. The maturation of the precursor particles into normal ribosomal subunits is completely blocked by cycloheximide. These results suggest that the maturation of the particles involves their transformation into mature ribosomal subunits which are joined to form the monomeric ribosomes, presumably, subsequent to their entrance into polyribosomes. © 1969.