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 Vla

The role of mRNA stability and translation in mediating the expression of s elected subunits of cytochrome c oxidase (COX) was examined during the diff erentiation of mouse myoblasts into myotubes in cell culture. The expressio n of the liver (L) and heart (H) isoforms of COX VIa, which undergo an isof orm switch during muscle development, as well as of the Va subunit, which i s 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 experime nts suggest that the translational efficiencies of these mRNAs do not chang e during myoblast differentiation, although the nuclear mRNAs for COX Va, V Ia-L and VIa-H are translated more efficiently than the mitochondrial mRNAs . Analysis of mRNA stability using the tetracycline-repressible promoter sy stem and/or actinomycin D indicates that COX VIa-L mRNA decays with a half- life of similar to 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 inst ability of COX Vla-L mRNA serves to limit the accumulation of COX VIa-L mRN A in these myogenic cells, as compared with mRNAs for other COX subunits. D eletion/replacement mapping experiments suggest that the COX VIa-L 3' untra nslated region contains a destabilization element. Analysis of the rate of poly(A) tail shortening on COX VIa-L and stable a-globin mRNAs suggests tha t the overall rate of poly(A) shortening per se is not rate limiting fbr th e degradation of COX VIa-L mRNA.