SUMMARY

• 38.1 Mature Ribosomal RNA Is Generated by the Cleavage of a Precursor Molecule

  RNA polymerase I transcribes the genes for three of the four rRNA molecules. The 18S RNA, 28S RNA, and 5.8S RNA are formed by the processing of an initial 45S transcript. In addition, the rRNAs are modified on the ribose and base components. Synthesis and processing take place in the nucleolus. The remaining rRNA, 5S RNA, is synthesized by RNA polymerase III.

• 38.2 Transfer RNA Is Extensively Processed

  Eukaryotic tRNAs are the products of RNA polymerase III. Both the 5′ and the 3′ ends are cleaved, and CCA is added to the 3′ end. Introns, present in some pre-tRNA molecules, are removed by an endonuclease and a ligase. Additionally, bases and riboses are modified.

• 38.3 Messenger RNA Is Modified and Spliced

  The 5′ ends of mRNA precursors become capped and methylated in the course of transcription. A 3′ poly(A) tail is added to most mRNA precursors after the nascent chain has been cleaved by an endonuclease.

  The splicing of mRNA precursors is carried out by spliceosomes, which consist of small nuclear ribonucleoprotein particles. Splice sites in mRNA precursors are specified by sequences at the ends of introns and by branch sites near their 3′ ends. The 2′-OH group of an adenosine residue in the branch site attacks the 5′ splice site to form a lariat intermediate. The newly generated 3′-OH terminus of the upstream exon then attacks the 3′ splice site to become joined to the downstream exon. Splicing thus consists of two transesterification reactions, with the number of phosphodiester linkages remaining constant during reactions. Small nuclear RNAs in spliceosomes catalyze the splicing of mRNA precursors. In particular, U2 and U6 snRNAs form the active centers of spliceosomes.

  The events in the posttranscriptional processing of mRNA are controlled by the phosphorylation state of the carboxyl-terminal domain, part of RNA polymerase II. RNA editing, which changes the mRNA nucleotide sequence, is another means of modifying mRNA.

• 38.4 RNA Can Function as a Catalyst

  Some RNA molecules, such as those containing the group I intron, undergo self-splicing in the absence of protein. A self-modified version of this rRNA intron displays true catalytic activity and is thus a ribozyme.