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Structure of a transcribing RNA polymerase II–U1 snRNP complex

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Science  15 Jan 2021:
Vol. 371, Issue 6526, pp. 305-309
DOI: 10.1126/science.abf1870

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A tight couple makes messenger RNAs

Gene expression in eukaryotes first requires transcription of DNA to an RNA copy and then splicing to form the final, processed messenger RNA (mRNA). Zhang et al. investigated how gene transcription and RNA splicing are physically coupled. Using cryo–electron microscopy, they resolved the molecular structure of a complex of the transcription enzyme RNA polymerase II with part of the splicing machinery, the U1 small nuclear ribonucleoprotein particle. The results provide important details for our understanding of coupled mRNA production.

Science, this issue p. 305

Abstract

To initiate cotranscriptional splicing, RNA polymerase II (Pol II) recruits the U1 small nuclear ribonucleoprotein particle (U1 snRNP) to nascent precursor messenger RNA (pre-mRNA). Here, we report the cryo–electron microscopy structure of a mammalian transcribing Pol II–U1 snRNP complex. The structure reveals that Pol II and U1 snRNP interact directly. This interaction positions the pre-mRNA 5′ splice site near the RNA exit site of Pol II. Extension of pre-mRNA retains the 5′ splice site, leading to the formation of a “growing intron loop.” Loop formation may facilitate scanning of nascent pre-mRNA for the 3′ splice site, functional pairing of distant intron ends, and prespliceosome assembly. Our results provide a starting point for a mechanistic analysis of cotranscriptional spliceosome assembly and the biogenesis of mRNA isoforms by alternative splicing.

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