U2 Binds Branch Site

FIGURE 12.09 Recognition of 5' Splice Site by U1 Snurp

A) The 5' splice site at the beginning of the intron is detected by base pairing with the RNA of the U1 snurp. B) The overall binding of U1 at the 5' splice site and of U2 at the branch site are shown.

The intron is removed and forms a loop with a branch. The exons are joined together.

called U2AF (U2 accessory factor) binds the 3' splice site. U4/U6 binds to U2 and U5 then arrives, binding first to the downstream exon and migrating to the intron/exon boundary. This makes the intron DNA loop appear as shown in Figure 12.10. The complex then rearranges itself. In particular U6 displaces U1 from the 5' splice site and U1 and U4 are lost from the complex.

Finally, splicing proceeds in two steps (Fig. 12.11). First, the intron and exon are cut apart at the 5' splice site and the free 5'-end of the intron loops around and is joined to the adenine at the branch site. Second, the free 3'-end of the upstream exon displaces the intron from the 3' splice site and the two exons are joined together. The intron is released as a branched lariat structure that is later degraded.

lariat structure Branched, lariat-shaped segment of RNA generated by splicing out an intron

U2AF (U2 accessory factor) Protein involved in splicing of introns that recognizes the down stream splice site

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