Construction and Identification by Partial Nucleotide Sequence Analysis of Bovine Casein and β-Lactoglobulin cDNA Clones

Double stranded (DS) DNA molecules obtained by reverse transcription of a partially purified lactating bovine mammary gland mRNA fraction were cloned into pBR322. Restriction maps for four recombinants were constructed and partial nucleotide sequence analysis of these revealed coding sequences corresponding to αs₁-, β-, and κ-casein and β-lactoglobulin. The specific single-stranded (SS) cDNAs representing each of these species were identified and their nucleotide lengths estimated. Evidence is presented that these are essentially full-length transcripts of the major mRNA species. On this basis, the cDNA clones range in size from 50% for β-casein to about 95% for α_s1-casein in comparison with their respective mRNAs. The DNA sequence spanning all eight phosphoserine residues in α_s1-casein is presented. These data, together with other serine codon usage data, indicate that the mammary gland phosphoseryl tRNA does not play a role in the incorporation of serine phosphate residues during casein synthesis. The observation that the nucleotide sequences for the serine phosphate cluster in bovine α_s1- and rat β-casein exhibit close homology supports the suggestion that these regions have evolved from a common primordial sequence.