THE PSEUDOURIDINE RESIDUES OF RIBOSOMAL-RNA

Pseudouridine (Psi), the most common single modified nucleoside in rib osomal RNA, has been positioned in the small subunit (SSU) and large s ubunit (LSU) RNAs of a number of representative species. Most of the i nformation has been obtained by application of a rapid primed reverse transcriptase sequencing technique. The locations of these Psi residue s have been compared. Many sites for Psi are the same among species, b ut others are distinct. In general, the percentage Psi in multicellula r eukaryotes is greater than in prokaryotes. In LSU RNA, the Psi resid ues are strongly clustered in three domains, all of which are near or connected to the peptidyl transferase center. There is no apparent clu stering of Psi in SSU RNA. The Psi sites in LSU RNA overlap those for the methylated nucleosides, but this is not the case in SSU RNA. There are 265 Psi sites known to nucleotide resolution, of which 246 are in defined secondary structures, and 112 of these are in nonidentical st ructural contexts. All 246 Psi sites can be classified into five struc tural types. Two Escherichia coli Psi synthases have been cloned and c haracterized, one for Psi 516 in SSU RNA and one for Psi 746 in LSU RN A. The Psi 746 synthase recognizes free RNA, but the Psi 516 enzyme re quires an intermediate RNP particle. Possible functional roles for Psi in the ribosome are discussed.