During pre-mRNA splicing, which small nuclear RNA (snRNA) component of the spliceosome is primarily responsible for recognizing and base-pairing with the branch point sequence (typically YNYURAC) in the intron?

Explanation

## Branch Point Recognition in Pre-mRNA Splicing **Key Point:** U2 snRNA is the spliceosomal component that recognizes and base-pairs with the branch point adenosine (the A in the YNYURAC consensus sequence) located 20–50 nucleotides upstream of the 3' splice site. ### The Spliceosome and Its snRNA Components | snRNA | Primary Function | Key Recognition Target | |-------|------------------|------------------------| | **U1** | Recognizes 5' splice site (GU consensus) | 5' splice site (GU...AG) | | **U2** | Recognizes branch point sequence | Branch point adenosine (YNYURAC) | | **U4** | Stabilizes U6; helps form catalytic core | Part of catalytic assembly | | **U5** | Recognizes exon sequences | Exon-intron boundaries | | **U6** | Forms catalytic core; positions substrates | Active site of spliceosome | ### The Branch Point Sequence **High-Yield:** The branch point sequence is a conserved purine-rich motif: - **Consensus**: YNYURAC (Y = pyrimidine, N = any nucleotide, R = purine, A = adenosine) - **Location**: 20–50 nucleotides upstream of the 3' splice site - **Critical residue**: The **adenosine (A)** in this sequence is the nucleophile that attacks the 5' splice site during the first transesterification reaction. ### Role of U2 snRNA 1. **Base pairing**: U2 snRNA forms Watson-Crick base pairs with the branch point sequence, stabilizing it. 2. **Adenosine activation**: This base pairing positions the branch point adenosine in the catalytic core of the spliceosome. 3. **Assembly**: U2 binding is one of the earliest steps in spliceosome assembly (E complex → A complex). **Clinical Pearl:** Mutations in the branch point sequence or U2 snRNA binding sites are a common cause of β-thalassemia and other splicing-defective genetic diseases. **Mnemonic:** **U2 → 2nd step** (U2 recognizes the branch point, which is used in the **second** transesterification reaction).