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Preribosomal RNA
Preribosomal RNA (pre-rRNA) represents a small class of RNA that is copied from DNA representing the genome sequence. However the pre-rRNA cannot be used for protein production until splicing of the introns occurs, forming a new bond between the exons and resulting in mature ribosomal RNA (rRNA).
==Overview==
During or immediately following transcription of pre-rRNA from rDNA in the nucleolus, the ribosomal RNA precursor (pre-rRNA) is modified and associates with a few ribosomal proteins.〔Marmier-Gourrier N, Cle´ry A, Schlotter F, Branlant C. A second base pair interaction between U3 small nucleolar RNA and the 5’-ETS region is required for early cleavage of the yeast pre-ribosomal RNA. Nucleic Acids Research. 2011; 39.〕 Small nucleolar RNAs (snoRNA) dictate the modifications, by base-pairing with target sites in eukaryotic pre-rRNA and may also play a role in pre-rRNA folding. Pre-rRNA contains external transcribed spacers (5'-ETS, 3' -ETS) at both ends as well as internal transcribed spacers (ITS1, ITS2). Cleavages at sites A’ and T1 remove the 5’-ETS and 3’-ETS, respectively. Cleavages at sites A0, 1 and 2 give rise to 18S rRNA. Site 3 cleavage can take place before or after cleavage at sites A0, 1, and 2 and may be responsible for the linkage between 18S and 28S rRNA processing pathways. The last steps of rRNA processing require cleavages at 3, 4’, 4 and 5 in order to generate mature 5.8S and 28S rRNA.
==Modifications==
Research suggests that either simultaneous to or immediately following synthesis of pre-rRNA, internal modifications are made at regions in the rRNA components, 18S, 5.8S, and 28S, which vary depending on cell type. Xenopus pre-rRNA modifications include ten base methylations, 105 2’-O-methylations of ribose and around 100 pseudouridines while yeast rRNA has merely half of these modifications.〔Gerbi SA, Borovjagin AV. Pre-Ribosomal RNA Processing in Multicellular Organisms. In: Madame Curie Bioscience Database (). Austin (TX): Landes Bioscience; 2000-.〕 Small nucleolar RNA base-pairs with the pre-rRNA and determines the site of modifications. Individual snoRNA families perform different modifications. Box C/D snoRNA guides the formation of 2’-O-Me, while Box H/ACA snoRNA guide the pseudouridines formation. There is thought that the base-pairing of snoRNA to pre-rRNA acts as a chaperone in the folding of mature rRNA.
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