Other RNA Processing Events and Post-Transcriptional Control of Gene Expression (CMB part 14)

Ribosomal RNAs are synthesized in the nucleoli of eukaryotic cells as precursors that require processing to yield mature rRNAs. The sequence of RNAs in the precursor is universally 18S, 5.8S, and 28S across all eukaryotes, although the precise sizes of the mature rRNAs differ among species. In human cells, the precursor is 45S, which undergoes a processing scheme that produces 41S, 32S, and 20S intermediates, with snoRNAs playing crucial roles in these steps. Extra nucleotides are removed from the 5'-ends of pre-tRNAs in a single step through endonucleolytic cleavage catalyzed by RNase P. Both bacterial and eukaryotic RNase P enzymes have a catalytic RNA subunit called M1 RNA. In E. coli, RNase II and polynucleotide phosphorylase cooperate to remove most of the additional nucleotides at the 3'-end of a tRNA precursor but halt at the 12-base stage. RNases PH and T are primarily responsible for removing the last two nucleotides. In eukaryotes, a single enzyme, tRNA 3'-processing endoribonuclease (3'-tRNase), performs the processing of the 3'-end of a pre-tRNA.Trypanosome mRNAs are generated through trans-splicing, which links a short leader exon with one of many independent coding exons. In trypanosomatid mitochondria, incomplete mRNAs require editing before translation. Editing occurs in the 3'→5' direction through sequential actions of one or more guide RNAs (gRNAs). These gRNAs bind to unedited mRNA regions, providing A's and G's as templates for inserting missing U's or deleting extra U's.In higher eukaryotes, including fruit flies and mammals, some adenosines in mRNAs must be post-transcriptionally deaminated to inosine for correct translation. This type of RNA editing is performed by enzymes called adenosine deaminases acting on RNAs (ADARs). Additionally, certain cytidines must be deaminated to uridine for accurate mRNA coding. Post-transcriptional gene regulation often involves such modifications to ensure proper gene expression.

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