Synthesis of the 60-S ribosomal subunit in yeast

Abstract We have previously shown that the process of ribosomal RNA synthesis in yeast is similar in many respects to that observed in other nucleated cells. This investigation concerns the formation of the large (60 S) subunit of the yeast ribosome. We demonstrate that a precursor to this subunit exists. The characteristics of this particle are: (1) it sediments at about 55 S in high-ionic-strength sucrose gradients; it sediments coincidentally with the 60-S subunit in low-ionic strength sucrose gradients; (3) the kinetics of its formation are those of a precursor particle; (4) it contains mature (25 S) ribosomal RNA; (5) the addition of protein to the mature ribosomal RNA takes longer than the processing of the ribosomal RNA precursor to 25-S RNA; and (6) the protein associated with the precursor particle is apparently obtained from a pool of ribosomal protein in the cell. We also show that this pool does not contain all the ribosomal proteins necessary for completion of the 60-S subunit.