Processing of the large rRNA precursor: two proposed categories of RNA-RNA interactions in eukaryotes.

The 5.8S RNA gene of eukaryotes is separated from the 26-28S rRNA gene by the internal transcribed spacer 2 (ITS 2). A compilation of known ITS 2 sequences is presented here. Four characteristic features of the ITS 2 primary structure are shared by all vertebrates. In contrast, lower eukaryotes lack most of these features, suggesting that the excision of the ITS 2 transcript during processing may differ between vertebrates and lower eukaryotes. Since the transcripts of rRNA ITS 2 and mRNA introns share some similarity, analogies have been made between the mechanisms of their removal during RNA maturation. A model is proposed for hydrogen-bonding of U3 snRNA with the 5' end of the vertebrate ITS 2 transcript. This U3 snRNA-ITS 2 RNA interaction does not appear to be used in ITS 2 processing in lower eukaryotes. Instead, in lower eukaryotes a region within the ITS 2 itself has the potential to hydrogen-bond to the 5' end of the ITS 2 transcript.