Violating the splicing rules: TG dinucleotides function as alternative 3' splice sites in U2-dependent introns.
BACKGROUND
Despite some degeneracy of sequence signals that govern splicing of eukaryotic pre-mRNAs, it is an accepted rule that U2-dependent introns exhibit the 3' terminal dinucleotide AG. Intrigued by anecdotal evidence for functional non-AG 3' splice sites, we carried out a human genome-wide screen.


RESULTS
We identified TG dinucleotides functioning as alternative 3' splice sites in 36 human genes. The TG-derived splice variants were experimentally validated with a success rate of 92%. Interestingly, ratios of alternative splice variants are tissue-specific for several introns. TG splice sites and their flanking intron sequences are substantially conserved between orthologous vertebrate genes, even between human and frog, indicating functional relevance. Remarkably, TG splice sites are exclusively found as alternative 3' splice sites, never as the sole 3' splice site for an intron, and we observed a distance constraint for TG-AG splice site tandems.


CONCLUSIONS
Since TGs splice sites are exclusively found as alternative 3' splice sites, the U2 spliceosome apparently accomplishes perfect specificity for 3' AGs at an early splicing step, but may choose 3' TGs during later steps. Given the tiny fraction of TG 3' splice sites compared to the vast amount of non-viable TGs, cis-acting sequence signals must significantly contribute to splice site definition. Thus, we consider TG-AG 3' splice site tandems as promising subjects for studies on the mechanisms of 3' splice site selection.
Szafranski
Karol
K
Genome Analysis, Leibniz Institute for Age Research, Fritz Lipmann Institute, Beutenbergstr, 07745 Jena, Germany. szafrans@fli-leibniz.de
Schindler
Stefanie
S
Taudien
Stefan
S
Hiller
Michael
M
Huse
Klaus
K
Jahn
Niels
N
Schreiber
Stefan
S
Backofen
Rolf
R
Platzer
Matthias
M
D001483:Base Sequence; D015894:Genome, Human; D006150:Guanine Nucleotides / Q000378:metabolism*; D006801:Humans; D007438:Introns; D008969:Molecular Sequence Data; D022821:RNA Splice Sites; D012342:RNA, Small Nuclear / Q000378:metabolism*; D017381:Spliceosomes / Q000378:metabolism; D013942:Thymine Nucleotides / Q000378:metabolism*
D006150:Guanine Nucleotides; D022821:RNA Splice Sites; D012342:RNA, Small Nuclear; D013942:Thymine Nucleotides; C405484:U2 small nuclear RNA