Associations of a Polymorphic AP-2 Binding Site in the 5′-Flanking Region of the Bovine β-Lactoglobulin Gene with Milk Proteins

摘要

Studies on a polymorphic position (R10) in an Activa-tor-Protein-2 (AP-2) binding site of the bovine β-Lacto-globulin (β-Lg) gene promoter region and quantitative traits of individual milk proteins were based on material from 79 German Holstein Friesian (HF) and 61 Simmen-tal (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-homozy-gotes. A similar association was found for a-lactalbumin, whereas the relative proportions and daily secreted amounts of caseins (α_(S1), 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.