The rate of secretion of αs2-casein into bovine milk is approximately 25% of that of β-casein, yet mammary expression of their respective mRNA transcripts (csn1s2 and csn2) is not different. Our objective was to identify molecular mechanisms that explain the difference in translation efficiency between csn1s2 and csn2. Cell-free translational efficiency of csn2 was 5 times that of csn1s2. Transcripts of csn1s2 distributed into heavier polysomes than csn2 transcripts, indicating an attenuation of elongation and/or termination. Stimulatory and inhibitory effects of the 5' and 3' UTRs on translational efficiency were different with luciferase and casein sequences in the coding regions. Substituting the 5' and 3' UTRs from csn2 into csn1s2 did not improve csn1s2 translation, implicating the coding region itself in the translation difference. Deletion of a 28-codon fragment from the 3' terminus of the csn1s2 coding region, which displays codons with low correlations to cell fitness, increased translation to a par with csn2. We conclude that the usage of the last 28 codons of csn1s2 is the main regulatory element that attenuates its expression and is responsible for the differential translational expression of csn1s2 and csn2.
αs2‐casein mRNA (csn1s2) is translated at 25% of the efficiency of β‐casein transcripts (csn2); however, the molecular mechanisms governing the difference are unknown. The main objective of this study was to identify molecular mechanisms that explain differential translational regulation between bovine β‐ and αs2‐ casein by assessing the role of putative translational regulatory factors in both cellular and cell‐free translation systems. Sequence analysis indicated that the two transcripts share similar primary and secondary structures around the coding region. Deleting and exchanging untranslated regions (UTRs) on the transcripts suggested that the 3′ UTR of csn2 and the 5′ UTR of csn1s2 exert stimulatory effects on translation yet their effectiveness depends on the upstream and downstream sequences with which they are associated. A stronger effect on translational efficiency was found in the coding region of csn1s2 which displays unfavourable codons at the 3′ terminus. Deletion of a 28‐codon fragment from the 3′ terminus of the csn1s2 coding region increased translation to a par with csn2. We conclude that the last 28 codons of csn1s2 is the main regulatory element that attenuates its expression and is responsible for the different translational expression of β‐ and αs2‐ casein mRNA.This research was supported by NSERC Canada.
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