In the first catalytic step of mRNA splicing, the 2' OH group of the bulged A at the branch site performs a nucleophilic attack on the 5' splice site phosphodiester bond, resulting in cleavage of the bond between the 5' exon and the 5' end of the intron, and formation of a new bond between the 5' end of the intron and the branch site A. This results in a lariat-shaped intermediate, with the intron still attached to the 3' exon. The branch site A has a 2'-5' phosphodiester bond with the G at the beginning of the intron, in addition to the usual 5'-3' and 3'-5'phosphodiester bonds.
Ilagan, JO, Chalkley, RJ, Burlingame, AL, Jurica, MS
Bessonov, S, Anokhina, M, Will, CL, Urlaub, H, Lührmann, R
Kataoka, N, Dreyfuss, G
Padgett, RA, Konarska, MM, Grabowski, PJ, Hardy, SF, Sharp, PA
Coltri, P, Effenberger, K, Chalkley, RJ, Burlingame, AL, Jurica, MS
Ruskin, B, Krainer, AR, Maniatis, T, Green, MR
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