Role of mRNA stability and translation in the expression of cytochrome c oxidase during mouse myoblast differentiation: instability of the mRNA for the liver isoform of subunit VIa

The role of mRNA stability and translation in mediating the expression of selected subunits of cytochrome c oxidase (COX) was examined during the differentiation of mouse myoblasts into myotubes in cell culture. The expression of the liver (L) and heart (H) isoforms of COX VIa, which undergo an isoform switch during muscle development, as well as of the Va subunit, which is expressed in all tissues, was analysed. The translational efficiencies of COX Va, VIa-L and VIa-H, as well as of mitochondrially encoded COX mRNAs, were inferred from their distribution in polysome gradients. These experiments suggest that the translational efficiencies of these mRNAs do not change during myoblast differentiation, although the nuclear mRNAs for COX Va, VIa-L and VIa-H are translated more efficiently than the mitochondrial mRNAs. Analysis of mRNA stability using the tetracycline-repressible promoter system and/or actinomycin D indicates that COX VIa-L mRNA decays with a half-life of ∼ 5–6h in both myoblasts and myotubes, whereas COX VIa-H and Va mRNAs decay with half-lives of > 15h in myotubes. This relative instability of COX VIa-L mRNA serves to limit the accumulation of COX VIa-L mRNA in these myogenic cells, as compared with mRNAs for other COX subunits. Deletion/replacement mapping experiments suggest that the COX VIa-L 3´ untranslated region contains a destabilization element. Analysis of the rate of poly(A) tail shortening on COX VIa-L and stable α-globin mRNAs suggests that the overall rate of poly(A) shortening per se is not rate limiting for the degradation of COX VIa-L mRNA.