Structural basis for hydrolytic splicing of a circularly permuted group II intron

Technical Summary: Structural Basis for Hydrolytic Splicing of a Circularly Permuted Group II Intron

Problem Statement
Group II introns are self-splicing ribozymes and evolutionary ancestors of the eukaryotic spliceosome. While canonical group II introns typically self-splice to generate linear exons and lariat intermediates, circularly permuted (CP) group II introns—identified in various bacterial phyla—exhibit distinct behaviors. These CP introns perform back-splicing to produce circular RNAs and branched products via a branching pathway. Crucially, they possess the capacity to switch to a hydrolysis pathway, yielding different products that may differentially regulate retrotransposition. Despite the biological relevance of these alternative pathways, the structural mechanisms governing the switch from the branching pathway to the hydrolysis pathway, and the specific structural rearrangements required for hydrolytic splicing, remained unresolved.

Methodology
This study employed a combination of biochemical assays and high-resolution cryogenic electron microscopy (cryo-EM) to investigate a CP group II intron from Comamonas testosteroni KF-1 (Cte 1). The researchers utilized these techniques to capture and reconstruct the structural states of the intron during both the branching and hydrolysis pathways. Specifically, the study focused on visualizing the active site rearrangements triggered by mutations in the branch point or splice sites (SS), as well as capturing intermediates of the second splicing step.

Key Results and Findings
The study provides high-resolution structures that allow for a mechanistic dissection of the splicing pathways:

• Mechanism of Pathway Switching: The structures reveal that the transition to the hydrolysis pathway is driven by mutations in the branch point or splice sites, which induce specific rearrangements within the active site.
• Active Site Rearrangement: In the hydrolysis pathway, the branching nucleotide located in domain D6 retracts from its catalytically competent conformation. This retraction creates space for a nucleophilic water molecule to attack the 5' splice site, initiating the hydrolysis reaction.
• Second-Step Intermediates: The researchers successfully visualized intermediates of the second splicing step. These structures demonstrate that domain D6 moves out of the way to facilitate 3'-splicing, a conformation that closely resembles the second-step structures observed in the canonical branching pathway.
• Scaffolding Role: The structural data provides direct evidence that domains D1 through D3 function as a structural scaffold supporting the group II intron architecture.

Significance and Claims
The paper claims to offer the first direct visualization of the complete hydrolysis pathway in CP group II introns. By elucidating the structural basis for the switch between branching and hydrolysis, the study enables a reconstruction of both steps of the hydrolysis pathway. The authors assert that these findings provide a new strategy for engineering CP group II introns specifically for the production of circular RNAs. The paper positions these insights as having potential utility in both basic research and therapeutic development, grounded in the structural understanding of how these ribozymes can be manipulated to favor specific splicing outcomes.