The casein mRNA decay changes in parallel with the poly(A) tail length in the mouse mammary gland.

Using beta- and gamma-casein mRNAs, the relationship between poly(A) tail length and half-life of mRNA is determined in the mouse mammary gland during pregnancy and lactation. beta- and gamma-Casein mRNAs increase before and after parturition, respectively. The poly(A) tail as well as the half-life of casein mRNA becomes longer upon the active casein mRNA synthesis. The poly(A) tail is shortened gradually as lactation progresses. The half-life of mRNA decreases approximately from 20 h at early to 4 h at late lactation. Northern blot analysis reveals that nuclear RNA has the same poly(A) tail length as casein mRNA in the cytoplasm does. Thus, the mammary gland changes the poly(A) tail length of casein mRNA. The poly(A) tail length changes in parallel with the level of poly(A) polymerase (PAP) mRNA during pregnancy and lactation, suggesting that the mammary gland determines the poly(A) tail length of casein mRNA through the change in the PAP gene expression. As the half-life of casein mRNA is related with the degree of polyadenylation, we conclude that the poly(A) tail elongation and shortening is a mechanism in regulating the mRNA decay.