Bovine αs2 -casein D is generated by exon VIII skipping

Bovine α s2 -casein D (CasD) differs from the common type A by the deletion of a stretch of 9 amino acids (aa) starting at a position not precisely known, either at aa 50, 51, or 52. The sequence of cloned PCR-amplified genomic DNA from three homozygous cows, two unrelated females carrying the CasD allele and one carrying the CasA allele, did not reveal any deletion and showed two identical nucleotide (nt) substitutions in the 1.7-kb region of both CasD alleles encompassing codons 43–75 in the cDNA encoding α s2 -CasA. This strongly suggests that the deleted bovine α s2 -CasD arises from skipping the 27-nt exon, now identified as exon VIII, which encodes aa 51–59 of α s2 -CasA. The G→T transversion (allele A→D) affecting the last nt of exon VIII, i.e., the 5' consensus splicing site, might be responsible for the altered splicing of the primary transcript of α s2 - CasD.