A SINGLE-POINT MUTATION RESULTS IN A-ALLELE-SPECIFIC EXON SKIPPING INTHE BOVINE ALPHA(S1)-CASEIN MESSENGER-RNA

Bovine alpha(s1)-casein (alpha(s1)-CN) allele A is found in low alleli c frequencies among different cattle breeds and is known to be charact erized by the deletion of amino-acid residues 14 to 26 of the mature p rotein (as defined via the most common allele B), and a corresponding deletion of 39 bp from its cDNA. Based upon the genomic sequence of bo vine alpha(s1)-CN [Koczan et al., Nucleic Acids Res. 19 (1991) 5591-55 96], this allelic deviation can be interpreted as an absence of exon 4 from the A allele mRNA and protein product. We demonstrate that this allelic aberration is not caused by a genomic deletion across the exon -4 DNA, but is correlated with a single point mutation at position +6 in the splice donor sequence distal of exon 4, which results in upstre am exon skipping during the serial splice reactions of the A allele al pha(s1)-CN pre-mRNA. The A-allele-specific mutation at position +6 is able to interrupt the perfect complementarity of the intron-4 splice d onor signal (positions one to eight) with U1-snRNA, which may then no longer be able to compensate for a rather weak exon-4 upstream splice acceptor sequence in facilitating the initial binding of U2 auxiliary factor/65-kDa (U2AF65) to that polypyrimidine tract. This interpretati on of the exon skipping mechanism in alpha(s1)-CN allele A is in agree ment with similar results obtained [Hoffmann and Grabowski, Genes Dev. 6 (1992) 2554-2568] in an analysis of the rat preprotachykinin-encodi ng gene and in vitro experiments.