Associations of a polymorphic AP-2 binding site in the 5′-flanking region of the bovine β-Lactoglobulin gene with milk proteins

Abstract

Studies on a polymorphic position (R10) in an Activator-Protein-2 (AP-2) binding site of the bovine β-Lactoglobulin (β-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 β-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 β-Lg secreted per day and proportion of β-Lg in milk protein compared with the C-homozygotes. A similar association was found for α-lactalbumin, whereas the relative proportions and daily secreted amounts of caseins (αS1, β, κ) showed lower values in β-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 β-Lg locus to a candidate gene for this trait. The association between the SNP in the AP-2 binding site of the β-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.