Discovery

The U1 snRNA was discovered in 1968 by Hodnett and Busch, who isolated the uridylic acid-rich RNA from rat liver cells and characterized it using PAGE. Throughout the following decades, it was discovered that U1 existed in a variety of organisms encompassing many kingdoms. Saccharmoyces cerevisiae U1 snRNA was discovered in the early 1980’s. Wise and colleagues, 1983, used northern and southern blots at low stringency and compared the results with U1 snRNP data collected from other organisms, such as molecular weights, presence of 5’ caps, etc.

Function

U1 snRNP is the first to bind the splicing complex to form the commitment complex during spliceosome assembly. The branch-point bridging protein then binds to a snRNP of U1 and the spliceosome cycle proceeds to the A complex. (Seraphin and Rosbash, 1989) The formation of the Commitment Complex is the initiation step for the whole spliceosome assembly, and is crucial. In Saccharmoyces cerevisiae, the U1 assembly is comprised of 16 U1 snRNP proteins, 7 Sm proteins, 9 U1 snRNP-specific proteins, and U1 A, C and 70K. The 5’ terminal sequence of U1 snRNA is complementary to the intron 5’ and 3’ splice site, and is highly conserved among eukaryotes, (Zhang and Rosbash, 1999) which suggests its importance in the initiation of splicing and the formation of the Commitment Complex.

The experimental technique used to determine the function of U1 was pretty remarkable. Yeast pre-mRNA was immobilized on agarose beads, and after adding yeast nuclear extract for varying periods of time, they observed via electrophoresis that at the earliest time, only U1 was bound to the anchored pre-mRNA (Ruby and Abelson, 1988).

Figure: Theoretical model of 2 human U1A snRNPs bound to pre-mRNA (2 views) (Jovine et al., 1996)

further information about this modelcan be found at the following site:
http://www.rcsb.org/pdb/cgi/explore.cgi?pid=12342989381560&page=0&pdbId=3UTR

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