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GAG triplets as splice acceptors of last resort. An unusual form of alternative splicing in prothymosin alpha pre-mRNA.

Prothymosin alpha pre-mRNAs are alternatively spliced as a consequence of adjacent AG acceptor couplets at the intron 2/exon 3 boundary of the only expressed human prothymosin alpha gene. These acceptors are found in a unique sequence motif, GAGGAG, located immediately 3' to a consensus polypyrimidine tract. The frequency with which each acceptor is utilized appears to be invariant in all human cells and tissues examined; two mRNA transcripts in the ratio of 9:1, shorter form: longer form, have been observed in every case. Production of the shorter mRNA violates two consensus rules for splice site selection: (1) the preferred AG dinucleotide is the second, rather than the first, following the polypyrimidine stretch; and (2) it lies in a potentially unfavorable, purine-rich region. The poor performance of the first AG dinucleotide cannot be explained by its position relative to other splicing signals; in mutant prothymosin alpha gene, this AG couplet promoted efficient splicing in vivo when preceded by a C residue or followed by a GAA triplet. The GAGGAG motif in prothymosin alpha genes has been retained by the African monkey, Colobus. Because the monkey's ancestors and our own diverged some 30 million years ago, the data suggest that the ambiguity in splice-site selection confers a selective advantage.