Formation of pseudouridine in U5 small nuclear RNA.

The formation of pseudouridine (psi) on U5 small nuclear RNA (U5 snRNA) was studied using an in vitro modification system. Labeled U5 RNA, synthesized in vitro and therefore unmodified, was incubated in reactions containing S100 and/or nuclear extracts (NE) from HeLa cells, and the levels of psi were determined. There are three psi residues found in human U5 RNA, at positions 43, 46, and 53. Incubation of unmodified U5 RNA in reactions containing either S100 or NE supports psi formation at positions 43 and 46, which are found in a loop in the predicted secondary structure of U5 RNA. However, psi formation at position 53, which is found in a stem, is dependent on the presence of NE during the incubation. The order of extract addition does not have a significant effect on the formation of psi at position 53 as long as NE is present. The most efficient psi formation was observed with a combination of S100 and NE which allowed for efficient small nuclear ribonucleoprotein particle (snRNP) assembly and psi formation. When 9S and 20S U5 snRNPs were isolated by velocity sedimentation gradient centrifugation after incubation in the combined extracts, there was little difference in the psi levels at any of the positions for the two distinct particles. Mutations in the U5 RNA sequence do affect psi formation. U5 RNAs that have mutated Sm binding sites or are truncated prior to the Sm binding site have very low levels of psi formation at positions 43 and 46 and no detectable psi formation at position 53. A deletion of five nucleotides from 39 to 43 abolishes psi formation at positions 43 and 46, but the modification of position 53 is unaffected.