Intron splicing: a conserved internal signal in introns of animal pre-mRNAs.

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Abstract
Splicing of introns of yeast pre-mRNA requires an internal conserved sequence T-A-C-T-A-A-C that is located 20-55 nucleotides from the 3'' intron boundary. Sequences differing only in certain positions from this yeast signal have now been identified in the corresponding internal region of pre-mRNA introns of a variety of animal genes. A computer program that searches for homologues to a consensus structure and calculates the accurary of match of each homologue is used to locate these sequences. The signals found by this search in introns of sea urchin, mouse, rat and human genes are listed and the consensus for each species is given. The consensus found for Drosophila and chicken and duck signals is also given. The accumulating evidence that these internal signals are required for splicing in animals is then discussed. It is also noted that a single-stranded region of small nuclear RNA U2 contains sequences complementary both to the proposed mammalian internal signal and to the neighboring .**GRAPHIC**. at the 3'' intron boundary. A role for U2 ribonucleoprotein in intron splicing is thus suggested.