UACUAAC is the preferred branch site for mammalian mRNA splicing.

The conserved branch-site sequence UAC-UAAC is known to form base pairs with the complementary sequence GUAGUA in U2 small nuclear RNA (snRNA) during mRNA splicing in the yeast Saccharomyces cerevisiae. Although the GUAGUA element is conserved in mammalian U2 snRNA, mammalian branch sites conform only weakly to a YURAC consensus and can even be deleted without obvious effects on the efficiency of splicing in vivo. To understand why the GUAGUA element of U2 is conserved in evolution but the branch site is not, we have devised two different competitive assays for branch-site selection using the first intron of the human beta-globin gene. We find that a sequence resembling UACUAAC is the most efficient branch site for mammalian mRNA splicing both in vivo and in vitro. Our results suggest that in mammals U2 snRNA can form base pairs with the branch site and the interaction between U2 and the branch site can be augmented or replaced by an interaction between the spliceosome and some other element of the intron or exons, perhaps the conserved polypyrimidine tract located immediately upstream from the 3' splice site.