Recurrent insertion of 5′-terminal nucleotides and loss of the branchpoint motif in lineages of group II introns inserted in mitochondrial preribosomal RNAs

A survey of sequence databases revealed 10 instances of subgroup IIB1 mitochondrial ribosomal introns with 1 to 33 additional nucleotides inserted between the 5′ exon and the consensus sequence at the intron 5′ end. These 10 introns depart further from the IIB1 consensus in their predicted domain VI structure: In contrast to its basal helix and distal GNRA terminal loop, the middle part of domain VI is highly variable and lacks the bulging A that serves as the branchpoint in lariat formation. In vitro experiments using two closely related IIB1 members inserted at the same ribosomal RNA site in the basidiomycete fungi Grifola frondosa and Pycnoporellus fulgens revealed that both ribozymes are capable of efficient self-splicing. However, whereas the Grifola intron was excised predominantly as a lariat, the Pycnoporellus intron, which possesses six additional nucleotides at the 5′ end, yielded only linear products, consistent with its predicted domain VI structure. Strikingly, all of the introns with 5′ terminal insertions lack the EBS2 exon-binding site. Moreover, several of them are part of the small subset of group II introns that encode potentially functional homing endonucleases of the LAGLIDADG family rather than reverse transcriptases. Such coincidences suggest causal relationships between the shift to DNA-based mobility, the loss of one of the two ribozyme sites for binding the 5′ exon, and the exclusive use of hydrolysis to initiate splicing.