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Previous studies suggested that domains 5 and 6 (D5 and D6) of group II introns act together in splicing and that the two helical structures probably do not interact by helix stacking. Here, we characterized the major Mg2+ ion- and salt-dependent, long-wave UV light-induced, intramolecular crosslinks formed in 4-thiouridine-containing D56 RNA from intron 5gamma (aI5gamma) of the COXI gene of yeast mtDNA. Four major crosslinks were mapped and found to result from covalent bonds between nucleotides separating D5 from D6 [called J(56)] and residues of D6 near and including the branch nucleotide. These findings are extended by results of similar experiments using 4-thioU containing D56 RNAs from a mutant allele of aI5gamma and from the group IIA intron, aI1. Trans-splicing experiments show that the crosslinked wild-type aI5gamma D56 RNAs are active for both splicing reactions, including some first-step branching. An RNA containing the 3-nt J(56) sequence and D6 of aI5gamma yields one main crosslink that is identical to the most minor of the crosslinks obtained with D56 RNA, but in this case in a cation-independent fashion. We conclude that the interaction between J(56) and D6 is influenced by charge repulsion between the D5 and D6 helix backbones and that high concentrations of cations allow the helices to approach closely under self-splicing conditions. The interaction between J(56) and D6 appears to be a significant factor establishing a side-by-side (i.e., not stacked) orientation of the helices of the two domains.

Photocrosslinking of 4-thio uracil-containing RNAs supports a side-by-side arrangement of domains 5 and 6 of a group II intron