Modification of human U4 RNA requires U6 RNA and multiple pseudouridine synthases

Area of Science:

• Molecular Biology
• RNA Modification
• Gene Splicing

Background:

• Small nuclear RNAs (snRNAs) are crucial for pre-mRNA splicing.
• These snRNAs undergo extensive post-transcriptional modifications.
• Pseudouridylation is a key modification affecting RNA structure and function.

Purpose of the Study:

• To investigate the modification of human U4 small nuclear RNA (snRNA).
• To identify factors and mechanisms involved in pseudouridine (Psi) formation in U4 snRNA.
• To elucidate the relationship between U4 snRNA modification and its association with U6 snRNA.

Main Methods:

• Utilized cell extracts and in vitro synthesized U4 snRNA.
• Employed micrococcal nuclease (MN) sensitivity assays to identify RNA cofactors.
• Fractionated extracts using glycerol gradients to isolate modifying activities.
• Used oligodeoxynucleotide-mediated RNase H digestion to assess U6 RNA's role.
• Applied 5-fluorouridine (5-FU) substituted U4 RNAs as competitive inhibitors.

Main Results:

• Pseudouridine formation in U4 snRNA depends on an RNA-containing cofactor sensitive to MN.
• Glycerol gradient analysis identified a 14S peak of reconstitution activity.
• Degradation of U6 RNA significantly inhibited Psi formation in U4 RNA.
• U4 RNA associated with U6 RNA (16S) showed higher Psi content than free U4 RNA (8S).
• Mutant U4 RNAs demonstrated differential inhibition of Psi synthases, indicating at least two modifying activities.

Conclusions:

• Pseudouridylation of U4 snRNA is mediated by an RNA cofactor and requires U6 snRNA association.
• At least two distinct enzymatic activities are responsible for modifying specific uridines in U4 snRNA.
• This study provides insights into the complex regulation of snRNA modification essential for splicing.