Cryo Electron Microscopy Structure Of LtrB With Its Reverse Transcriptase

Group II introns,like nuclear spliceosomal introns,splice through successive transesterification reactions in which two nucleophilic attacks give rise to an excised intron lariat and ligated exons.Whereas nuclear intron splicing is catalyzed by a spliceosome,a complex and dynamic ribonucleoprotein(RNP)complex consisting of small nuclear RNAs and multiple protein factors,group II intron splicing is catalyzed by the intron ribozyme and a single intron-encoded protein(IEP).The IEP has maturase activity,which facilitates splicing,and reverse transcriptase(RT)activity,which as in other retroelements can mobilize the intron by a target primed reverse transcription(TPRT)mechanism.Once excised from the primary transcript,the intron RNA can reverse splice into DNA,utilizing the IEP to complete the retromobility reaction.Group II intron RNA consists of six structural domains and the IEP binds to the intron RNA after its translation from the Domain IV,to form a ribonucleoprotein(RNP)complex,which promotes intron splicing.However,structural studies to date have not yielded insights into intact RNPs,rather into portions of the spliceosome and only proteinfree group II intron ribozymes.To enhance our understanding of these reactions,defining the structure of a group II intron RNP complex is necessary.We therefore chose the LtrB intron from Lactococcus lactis,being the best characterized group II intron RNP,with an IEP that contains maturase and RT activities as well as endonuclease needed for IEP-facilitated TPRT.Here we report cryo-EM structures of LtrB intron in its ribonucleoprotein complex form at 3.8 ? resolution and in its protein-depleted form at 4.5 ? resolution,revealing functional coordination of the intron RNA with the protein.Remarkably,the protein structure reveals a close relationship of the reverse transcriptase catalytic domain to telomerase,whereas the active center for splicing resembles the spliceosomal Prp8 protein and splicing center.These extraordinary similarities hint at intricate ancestral relationships and provide new insights into splicing and retromobility.