Widespread exon definition is promoted by the U1 snRNP 70K subunit and requires sites of contact with RNA Polymerase II

Recent research elucidates a key mechanism in pre-mRNA splicing, revealing that a subunit of U1 snRNP plays a pivotal role in the process. Scientists delved into this phenomenon by depleting the U1-70K subunit, which interacts with RNA polymerase II (RNAPII) during transcription. The findings indicate that this depletion leads to extensive exon skipping, suggesting a widespread method of exon definition.

Contrary to expectations, the exons with the most robust splice sites (SSs) are the ones most likely to be skipped. Moreover, modifying non-canonical SSs or using a drug that promotes similar SSs resulted in exon skipping when U1-70K was missing. This discovery points to a previously unrecognized function of U1-70K: destabilizing the interaction between U1 and the 5 SS, paving the way for the transition to the U6-5 SS necessary for efficient splicing following exon definition.

Further support for this theory came from observing that knocking down Prp28/DDX23, which facilitates U1-U6 exchange at the 5 SS, also induces exon skipping, mirroring the effects of U1-70K depletion. Additionally, swapping functional U1-70K with a mutant lacking the RNAPII interface in cells with degraded U1-70K induced widespread exon skipping. The authors propose that this process unfolds at the transcription elongation complex and is facilitated by the contact between U1-70K and RNAPII.