Cryo-EM Structures of a Group II Intron Reverse Splicing into DNA.

Group II introns are a class of retroelements that invade DNA through a copy-and-paste mechanism known as retrotransposition. Their coordinated activities occur within a complex that includes a maturase protein, which promotes splicing through an unknown mechanism. The mechanism of splice site exchange within the RNA active site during catalysis also remains unclear. We determined two cryo-EM structures at 3.6-Å resolution of a group II intron reverse splicing into DNA. These structures reveal that the branch-site domain VI helix swings 90°, enabling substrate exchange during DNA integration. The maturase assists catalysis through a transient RNA-protein contact with domain VI that positions the branch-site adenosine for lariat formation during forward splicing. These findings provide the first direct evidence of the role the maturase plays during group II intron catalysis. The domain VI dynamics closely parallel spliceosomal branch-site helix movement and provide strong evidence for a retroelement origin of the spliceosome.

Recognition of transcription terminators during retrotransposition: How to keep a group II intron quiet.

Chung et al. recently presented the structure of a primitive group IIC intron with its DNA target, which reveals the structural requirements that this class of intron uses to recognize a transcription terminator stem loop at the DNA level for insertion during retrotransposition.

Structural basis of branching during RNA splicing.

Branching is a critical step in RNA splicing that is essential for 5' splice site selection. Recent spliceosome structures have led to competing models for the recognition of the invariant adenosine at the branch point. However, there are no structures of any splicing complex with the adenosine nucleophile docked in the active site and positioned to attack the 5' splice site. Thus we lack a mechanistic understanding of adenosine selection and splice site recognition during RNA splicing. Here we present a cryo-electron microscopy structure of a group II intron that reveals that active site dynamics are coupled to the formation of a base triple within the branch-site helix that positions the 2'-OH of the adenosine for nucleophilic attack on the 5' scissile phosphate. This structure, complemented with biochemistry and comparative analyses to splicing complexes, supports a base triple model of adenosine recognition for branching within group II introns and the evolutionarily related spliceosome.

Scaffold-enabled high-resolution cryo-EM structure determination of RNA.

Cryo-EM structure determination of protein-free RNAs has remained difficult with most attempts yielding low to moderate resolution and lacking nucleotide-level detail. These difficulties are compounded for small RNAs as cryo-EM is inherently more difficult for lower molecular weight macromolecules. Here we present a strategy for fusing small RNAs to a group II intron that yields high resolution structures of the appended RNA, which we demonstrate with the 86-nucleotide thiamine pyrophosphate (TPP) riboswitch, and visualizing the riboswitch ligand binding pocket at 2.5 Å resolution. We also determined the structure of the ligand-free apo state and observe that the aptamer domain of the riboswitch undergoes a large-scale conformational change upon ligand binding, illustrating how small molecule binding to an RNA can induce large effects on gene expression. This study both sets a new standard for cryo-EM riboswitch visualization and offers a versatile strategy applicable to a broad range of small to moderate-sized RNAs, which were previously intractable for high-resolution cryo-EM studies.

Retroelement origins of pre-mRNA splicing.

Recent cryo-EM structures of a group II intron caught in the process of invading DNA have given new insight into the mechanisms of both splicing and retrotransposition. Conformational dynamics involving the branch-site helix domain VI are responsible for substrate exchange between the two steps of splicing. These structural rearrangements have strong parallels with the movement of the branch-site helix in the spliceosome during catalysis. This is strong evidence for the spliceosome evolving from a group II intron ancestor. We observe other topological changes in the overall structure of the catalytic domain V that may occur in the spliceosome as well. Therefore, studying group II introns not only provides us with insight into the evolutionary origins of the spliceosome, but also may inform the design of experiments to further probe structure-function relationships in this eukaryotic splicing apparatus. This article is categorized under: RNA Processing > Splicing Mechanisms RNA Structure and Dynamics > RNA Structure, Dynamics, and Chemistry RNA Structure and Dynamics > Influence of RNA Structure in Biological Systems RNA Evolution and Genomics > RNA and Ribonucleoprotein Evolution.