A unique subspecies of histone H4 mRNA from rat myoblasts contains poly(A).

Fractionation of rat L6 myoblast histone H4 mRNA into its three component subspecies revealed that one of the major subspecies (H4-1) contained poly(A). The unique poly(A)+ H4 mRNA makes up about 8% of the total polysomal H4 mRNA population detected. Unlike the poly(A)- histone mRNAs, whose levels are reduced by greater than 95% when myoblasts differentiate into myotubes, the poly(A)+ subspecies is reduced by only 70%. The poly(A)+ H4 mRNA from myotubes incubated with actinomycin D decays with a half-life of 37-42 min, which is similar to that obtained for the poly(A)- H4 mRNAs in myoblasts. Both the poly(A)+ and poly(A)- subspecies decay at an increased rate after inhibition of DNA synthesis. In myoblasts the poly(A)+ H4 mRNA exists almost exclusively in the polysomal compartment (greater than 95%) with little (less than 5%) in the free ribonucleoprotein (mRNA-protein or mRNP) complex compartment of the cell. Poly(A)- histone H4 mRNA subspecies, on the other hand, are distributed with approximately 80% in the polysomal compartment and 20% in the free mRNP complex compartment. The unique poly(A)+ H4 mRNA is unusual, not only in that it contains poly(A) but also in its behavior compared to poly(A)- H4 mRNAs during terminal differentiation.