| Abstract
| - Group II introns encode proteins with reverse transcriptase activity. These proteins also promoteRNA splicing (maturase activity) and then, with the excised intron, form a site-specific DNA endonucleasethat promotes intron mobility by reverse splicing into DNA followed by target DNA-primed reversetranscription. Here, we used an Escherichia coli expression system for the Lactococcus lactis group IIintron Ll.LtrB to show that the intron-encoded protein (LtrA) alone is sufficient for maturase activity,and that RNP particles containing only the LtrA protein and excised intron RNA have site-specific DNAendonuclease and target DNA-primed reverse transcriptase activity. Detailed analysis of the splicing reactionindicates that LtrA is an intron-specific splicing factor that binds to unspliced precursor RNA with a Kdof ≤0.12 pM at 30 °C. This binding occurs in a rapid bimolecular reaction, which is followed by a slowerstep, presumably an RNA conformational change, required for splicing to occur. Our results constitutethe first biochemical analysis of protein-dependent splicing of a group II intron and demonstrate that asingle intron-encoded protein can interact with the intron RNA to carry out a coordinated series of reactionsleading to splicing and mobility.
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