Sequence and single-base polymorphisms of the bovine alpha-lactalbumin 5'-flanking region

The alpha-lactalbumin (alpha LA)-encoding gene is a potential quantitative trait locus in dairy animals. In cattle, the production of alpha LA is tightly coupled to the onset of lactation and it serves as a regulatory subunit of the enzyme responsible for lactose synthesis. Lactose is the major osmole controlling water movement in the mammary gland. To better understand the control of bovine alpha LA expression, the 5'-flanking region of a Holstein alpha LA gene was cloned and sequenced. The sequenced clone contains 1952 bp of 5'-flanking region and 66-bp of the protein-coding region. Three single-bp polymorphisms were identified within this region. These polymorphisms occur at positions +15, +21 and +54 relative to the mRNA transcription start point (tsp). The +15 and +21 variations occur in the region encoding the 5'-untranslated region of the mRNA-coding sequence. The +54 polymorphism is a silent mutation in the SP-coding region of the gene. A polymerase chain reaction (PCR, Cetus)-based screening method has been employed to analyze the genotype of cattle at the +15 position. A total of 501 randomly selected cattle from seven breeds were screened for this allele. Of these animals, only the Holstein breed of cattle was found to contain the +15 variation and it occurs at a gene frequency of 32%. Sequence comparisons were conducted between the 5'-flanking regions of the bovine-milk-protein encoding genes, alpha LA, beta-casein and alpha S1-casein, which are coordinately expressed. Regions of similarity extending to 350 bp in length were observed between these sequences.