Structure of a transcribing RNA polymerase II-U1 snRNP complex

Area of Science:

• Molecular Biology
• Structural Biology
• RNA Processing

Background:

• Cotranscriptional splicing is crucial for gene expression, involving RNA polymerase II (Pol II) and U1 small nuclear ribonucleoprotein particle (U1 snRNP).
• Understanding the structural basis of Pol II-U1 snRNP interaction is key to elucidating early steps of spliceosome assembly.

Purpose of the Study:

• To determine the structure of the mammalian transcribing Pol II-U1 snRNP complex.
• To elucidate the mechanism by which Pol II facilitates cotranscriptional splicing initiation.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was used to visualize the mammalian transcribing Pol II-U1 snRNP complex.
• Structural analysis of the complex to identify direct interaction interfaces.

Main Results:

• Direct interaction between Pol II and U1 snRNP was observed.
• This interaction positions the pre-mRNA 5' splice site near the Pol II RNA exit site.
• Formation of a "growing intron loop" during pre-mRNA extension was revealed, facilitating downstream splicing events.

Conclusions:

• The study provides the first structural insights into the mammalian transcribing Pol II-U1 snRNP complex.
• The findings offer a mechanistic basis for cotranscriptional spliceosome assembly.
• This work lays the groundwork for understanding mRNA isoform biogenesis through alternative splicing.