Single-particle cryo-electron microscopy (cryo-EM) is an emerging technique in the field of structural biology. With the recent advancement in electron detection technology and improved image processing algorithms, now it is possible to achieve atomic resolution structure of macromolecular complexes. We have been applying cryo-EM to illustrate the architecture of ribonucleoprotein complexes, such as bacterial group II introns and ribosomes. Bacterial group II introns are large catalytic RNAs related to nuclear spliceosomal introns and eukaryotic retrotransposons. They self-splice, yielding mature RNA, and integrate into DNA as retroelements. A fully active group II intron forms a ribonucleoprotein complex comprising the intron ribozyme and an intron-encoded protein that performs multiple activities including reverse transcription, in which intron RNA is copied into the DNA target. Recently, the first cryo-EM structure of Lactococcus lactis group IIA intron, in complex with an intron-encoded protein, was resolved at 3.8 Å resolution and its protein-depleted form at 4.5-Å resolution. Molecular analysis of the cryo-EM structure of the group II intron reveals functional coordination of the intron RNA with the protein. Interestingly, the protein structure revealed a close relationship between the reverse transcriptase catalytic domain and telomerase, whereas the active splicing center resembles the spliceosomal Prp8 protein. These extraordinary similarities hint at intricate ancestral relationships and provide new insights into splicing and retromobility. I will highlight the recent development in the cryo- EM field and summaries our findings on group II introns.
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