The spacing between functional Cis-elements of U3 snoRNA is critical for rRNA processing

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Ribosome biogenesis is a fundamental cellular process involving the processing of pre-ribosomal RNA (pre-rRNA).
• Small nucleolar RNAs (snoRNAs), particularly U3 snoRNA, play critical roles in guiding site-specific cleavage events during pre-rRNA maturation.
• Understanding the structural and functional elements of U3 snoRNA is vital for elucidating the mechanisms of ribosome assembly.

Purpose of the Study:

• To investigate the specific sequence and structural requirements of Xenopus laevis U3 small nucleolar RNA (snoRNA) for pre-rRNA cleavage at sites 1 and 2.
• To determine the functional importance of different domains and elements within U3 snoRNA, including domain I, the 5' hinge, 3' hinge, and boxes A' and A, in the formation of 18 S ribosomal RNA (rRNA).

Main Methods:

• Depletion/rescue experiments were performed in Xenopus oocytes to assess the function of U3 snoRNA variants.
• Site-directed mutagenesis was employed to create alterations in specific regions of U3 snoRNA, including domain I, hinge regions, and boxes A' and A.
• Analysis of pre-rRNA processing intermediates and products was conducted to evaluate the impact of U3 snoRNA mutations.

Main Results:

• The putative stem of U3 domain I was found to be unnecessary for 18 S rRNA processing.
• A newly discovered 5' hinge element in Xenopus U3 snoRNA was identified as important, though less critical than the 3' hinge region, for 18 S rRNA formation.
• Direct experimental evidence confirmed that box A' is essential for U3 snoRNA function in 18 S rRNA formation, and the spacing between U3 elements is critical, suggesting simultaneous base-pairing with pre-rRNA.

Conclusions:

• The structure of U3 domain I, when unbound to pre-rRNA, may differ from its bound state.
• Phylogenetically conserved base-pairing interactions between U3 snoRNA hinge regions (5' and 3') and the external transcribed spacer (ETS) of pre-rRNA are crucial for positioning U3 boxes A'+A.
• U3 snoRNA boxes A' and A likely base-pair with 18 S rRNA sequences, with box A' being indispensable for U3 snoRNA's role in 18 S rRNA biogenesis.