Multiple mRNA species code for two non‐allelic forms of ovine αs2‐casein

Open Access
Publisher Website
Google Scholar
Abstract
The two non-allelic forms of αs2-casein, occurring in ovine milk, differ by an internal deletion of nine amino acid residues, at positions 34 and 42 in the mature chain. Sequencing of several αs2-casein cDNA, isolated from the mammary cDNA library of a single lactating ewe, showed three new types which differed from that previously studied. In addition to the expected deletion of codons + 34 to + 42 affecting 30–40% of mRNA, another structural difference involving an internal stretch of 44 nucleotides in the 5′untranslated region, was found. S1-nuclease protection assays confirmed the existence of several types of the relevant mRNA and sequencing of in-vitro-amplified genomic DNA demonstrated the presence of the 44-nucleotide stretch in the αs2-casein transcriptional unit, thus ruling out the possibility of a cloning artefact. The different αs2-casein mRNA, four in terms of deletion and two in terms of nucleotide substitutions for a given ewe, can be readily explained by partial exon skipping and allelic differences, respectively. This assumption is well supported by the following observations: 5′ and 3′ ends of both deleted DNA fragments are similar to those of exons; sequences neighbouring the 44-nucleotide stretch of the genomic DNA perfectly match consensus sequences described for 3′ and 5′ ends of introns; the rather simple patterns observed on Southern blots of different enzymatic digests of genomic DNA strongly suggest the occurrence of only 1 copy αs2-casein gene/haploid genome. During the course of evolution, the αs2-casein-encoding gene has undergone many mutations and some of them might have occurred in regions corresponding to consensus splicing regions of the pre-mRNA. Thus, complete skipping of some exons might be responsible for the shorter sizes of rat and mouse αs2-casein mRNA. If so, the overall organization of the αs2-casein gene in the different species might be more similar than expected from structural comparisons of the cognate mRNA or caseins.