ABSTRACT
RNA processing describes the structural and chemical maturation of newly synthesized RNA molecules. The modifications occur during transcription (cotranscriptional modification) and afterwards (posttranscriptional modification); they may be essential for RNA function or may represent a mechanism of gene regulation. Such reactions fall into ten categories, as shown in Table 27.1. Categories of RNA processing reactions Processing reaction Examples Cleavage Release of rRNA and tRNA from polycistronic transcripts Termination of eukaryotic mRNA transcription Exonucleolytic degradation Processing rRNA and tRNA to generate mature ends Nucleotidyl transfer Transfer of CCA trinucleotide to 3′ end of some tRNAs Chemical modification of bases Occasional methylation in mRNA and rRNA Extensive base modification in tRNA and snRNA Nucleotide excision and replacement Hypermodification of guanosine residues in tRNA to produce queuosine and wyosine Capping Addition of 7-methylguanosine to 5′ end of eukaryotic mRNA Polyadenylation Addition of polyadenylate tail to 3′ end of most eukaryotic mRNAs and a few bacterial mRNAs Splicing (tansesterification) Removal of most introns (usually in cis, but occasionally in trans) Splicing (ligation) Removal of tRNA introns Editing Changing the information carried in mRNA by base modification, insertion and deletion of residues from the coding region
