Studies of point mutants define three essential paired nucleotides in the domain 5 substructure of a group II

Domain 5 (D5) is a highly conserved, largely helical substructure of group II introns that is essential for self-splicing. Only three of the 14 base pairs present in most D5 structures (A2 U33, G3 U32, and C4 G31) are nearly invariant. We have studied effects of point mutations of those six nucleotides on self-splicing and in vivo splicing of aI5g, an intron of the COXI gene of Saccharomyces cerevisiae mitochondria. Though none of the point mutations blocked self-splicing under one commonly used in vitro reaction condition, the most debili-tating mutations were at G3 and C4. Following mitochondrial Biolistic transformation, it was found that mutations at A2, G3, and C4 blocked respiratory growth and splicing while mutations at the other sites had little effect on either phenotype. Intra-D5 second-site suppressors showed that pairing between nucleotides at positions 2 and 33 and 4 and 31 is especially important for D5 function. At the G3 U32 wobble pair, the mutant A U pair blocks splicing, but a revertant of that mutant that can form an A1 C base pair regains some splicing. A dominant nuclear suppressor restores some splicing to the G3A mutant but not the G3U mutant, suggesting that a purine is required at position 3. These findings are discussed in terms of the hypothesis of Madhani and Guthrie (H. D. Madhani and C. Guthrie, Cell 71:803–817, 1992) that helix 1 formed between yeast U2 and U6 small nuclear RNAs may be the spliceosomal cognate of D5. Self-splicing group II introns have conserved secondary-ter