PROTEIN-INDUCED FOLDING OF A GROUP-I INTRON IN CYTOCHROME-B PRE-MESSENGER-RNA

Some group I introns have been shown to be self-splicing in vitro, but perhaps all require proteins for splicing in vivo, Sequence differenc es affect the stability of secondary structures and may explain why so me group I introns function efficiently without protein cofactors whil e others require them. The terminal intron of the cytochrome b pre-mRN A from yeast mitochondria needs a nucleus encoded protein for splicing , even though it splices autocatalytically in high salt in vitro. This system has the advantage that the protein is specific for this intron , and yet the structure of the catalytically active RNA can be studied in its absence, We have modified the intron by chemical and enzymatic treatment in the presence and absence of the protein to determine the impact of the protein on the secondary and tertiary structures of the intron, We found protein-induced formation of secondary and tertiary structures within the intron, and the same structures also form in hig h salt autocatalytic conditions, We have also studied UV cross-links t o determine those bases of the intron that interact directly with the protein and found that the protein contacts the intron most intimately at the structures denoted P1, L2, P4, and P6a.