Processing of rRNA and tRNA
■ A large precursor pre-rRNA (45S in humans) synthesized by RNA polymerase I undergoes cleavage, exonucle-
olytic digestion, and base modifications to yield mature 28S, 18S, and 5.8S rRNAs, which associate with riboso-mal proteins into ribosomal subunits (see Figure 12-34).
■ Synthesis and processing of pre-rRNA occur in the nucleolus. The 5S rRNA component of the large ribosomal subunit is synthesized in the nucleoplasm by RNA polymerase III and is not processed.
■ Group I and group II self-splicing introns and snRNAs in spliceosomes all function as ribozymes, or catalytically active RNA sequences, that carry out splicing by analogous transesterification reactions requiring bound Mg+2 ions (see Figure 12-35).
■ Pre-tRNAs synthesized by RNA polymerase III in the nucleoplasm are processed by removal of the 5'-end sequence, addition of CCA to the 3' end, and modification of multiple internal bases (see Figure 12-36).
■ Some pre-tRNAs contain a short intron that is removed by a protein-catalyzed mechanism distinct from the splicing of pre-mRNA and self-splicing introns.
■ All species of RNA molecules are associated with proteins in various types of ribonucleoprotein particles both in the nucleus and after export to the cytoplasm.
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