Polymorphic AP-1 Binding Site in Bovine CSN1S1 Shows Quantitative Differences in Protein Binding Associated with Milk Protein Expression

Polymorphisms in 5′-flanking regions of milk protein encoding genes can influence the binding activity of the affected response elements and thus have an impact on the expression of the gene products. However, precise quantitative data concerning the binding properties of such variable response elements have so far not been described. In this study we present the results of a quantitative fluorescent electromobility shift assay comparing the allelic variants of a polymorphic activator protein-1 binding site in the promoter region of the bovine αs1-casein encoding gene (CSN1S1), which is affected by an A→G exchange at −175 bp (CSN1S1−175bp). A supershift assay using a commercial c-jun antibody was carried out to verify the specificity of protein binding. The gel shift analysis revealed specific and significantly reduced protein binding of oligonucleotides containing the G variant of the CSN1S1−175bp binding site. Further investigations comprised genotyping of the variable CSN1S1−175bp activator protein-1 element by an NmuCl restriction fragment length polymorphism in 62 cows of the breed Simmental and 80 cows of the breed German Holstein. Single milk proteins from at least 4 milk samples per cow were quantified by alkaline urea polyacrylamide gel electrophoresis. Homozygotes for CSN1S1−175bp*G were not observed, and the allele frequencies were 0.19 in Simmental and 0.05 in German Holstein. Carriers of CSN1S1−175bp*G showed higher content (%) as well as quantity (g/d) of αs1-casein than CSN1S1−175bp*A homozygotes, independent of breed. We assume that the positive association of the CSN1S1−175bp*G variant with CSN1S1 expression is likely to be caused by a reduced affinity of the affected response element to a c-jun-containing CSN1S1 dimer with repressor properties.