Point mutations identify the conserved, intron-contained TACTAAC box as an essential splicing signal sequence in yeast.

Our previous deletion experiments have shown that a short region of yeast nuclear gene introns containing the conserved sequence 5'-TACTAACA-3' is essential for splicing. In this report we show that the chemically synthesized decanucleotide 5'-TGTACTAACA-3', when introduced into a hybrid gene forming unspliceable RNA molecules, results in the generation of spliceable transcripts. Single A----C transversions in the fourth or eighth position of this sequence eliminated its intron-generating capacity. The C----T transition in the fifth position, generated by sodium bisulphite mutagenesis, did not affect the efficiency and accuracy of splicing. These results clearly demonstrate the biological significance of this conserved intron sequence and shed further light on its possible functioning.