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).

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. 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.

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. 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.

Pre-rRNA comprise three main sizes; 37S (yeast), 40S (Xenopus) and 45S (mammals). In a series of steps, nearly 80 ribosomal proteins assemble with the pre-rRNA. During transcription of pre-rRNA, early ribosomal binding proteins associate. It is thought that this 30S RNP containing 45S pre-rRNA is the precursor for 80S RNP, which in turn, is the precursor to 55S RNP. 55S RNP makes up ~75% of the nucleolar population of pre-ribosomes.

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