Associations of a polymorphic AP-2 binding site in the 5'-flanking region of the bovine beta-lactoglobulin gene with milk proteins.

Studies on a polymorphic position (R10) in an Activator-Protein-2 (AP-2) binding site of the bovine beta-Lactoglobulin (beta-Lg) gene promoter region and quantitative traits of individual milk proteins were based on material from 79 German Holstein Friesian (HF) and 61 Simmental (Sm) cows. At least four milk samples per cow were analyzed with alkaline Urea-PAGE in combination with densitometry for quantification of individual milk proteins. The two alleles of the R10 single nucleotide polymorphism (SNP) carry either G or C in position -435 bp of the beta-Lg promoter region. G- and C-alleles were found in Sm with nearly equal frequencies, while in HF the C-allele frequency was higher (0.73) than that of the G-allele. In both breeds, the R10 G-homozygotes had higher (P < 0.001) amounts of beta-Lg secreted per day and proportion of beta-Lg in milk protein compared with the C-homozygotes. A similar association was found for alpha-lactalbumin, whereas the relative proportions and daily secreted amounts of caseins (alphaS1, beta, kappa) showed lower values in beta-Lg R10 G-homozygotes. A positive association (P < 0.001) of R10 CC with milk yield has also been observed and indicates a close proximity of the beta-Lg locus to a candidate gene for this trait. The association between the SNP in the AP-2 binding site of the beta-Lg gene and its gene product can be explained as the result of differences in protein binding activity, and, therefore, allele specific differences in gene expression. Thus, our study clearly links a DNA polymorphism of molecular function very closely with in vivo expression parameters of the same locus.