The nucleotide sequences corresponding to bovine αS1- and k-casein mRNAs are presented. An unusual αS1-casein cDNA has been characterised whose 5′ end commences upstream from its putative TATA box. The αS1- casein mRNA is compared to rat αS1-casein mRNA and two components of divergence are identified. Firstly, the two sequences have diverged at a high point mutation rate and the rate of amino acid replacement by this mechanism is at least as great as the rate of divergence of any other part of the mRNAs. Secondly, the protein coding sequence has been subjected to several insertion/deletion events, one of which may be an example of exon shuffling. The x-casein mRNA sequence verifies the proposition that it has arisen from a different ancestral gene to the other caseins.
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