Molecular Guardian of RNA (IMAGE) University of Pennsylvania School of Medicine Caption From gene expression to protein. Pre-mRNA is the direct transcript of a gene, but before it is made into proteins, it needs to be processed to form messenger RNA (mRNA), including splicing of introns and joining of exons, as well as cleavage/polyadenylation at the end of the transcript. The splicing reaction is mediated by the spliceosome, usually comprised of U1, U2, U4/U5 and U6 snRNPs that come together on each intron. U1, an abundant component of the splicing machinery, binds to the 5' splice site of introns for splicing, but in addition, as the Nature study shows, it also protects pre-mRNAs from premature termination. Credit The Dreyfuss Laboratory; University of Pennsylvania School of Medicine Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.