Cytoplasmic distribution of pulse-labelled poly(A)-containing RNA, particularly 26 S RNA, during myoblast growth and differentiation

Abstract

Total poly(A)-containing RNA in different polysomal and supernatant cytoplasmic fractions was analysed after pulse-labelling in dividing myoblasts and fused myotubes. In particular, the peak of 26 S RNA (putative messenger for the large subunit of myosin) is located in a light region of the gradient coinciding with the monosome-trisome fractions prior to fusion, and is found in the heavy polysomes only after fusion. These heavy polysomes are free (i.e. not membrane bound). Treatment of the light part of the polysome gradient with EDTA shows that the 26 S RNA found here does not exist as part of a polysomal complex, but is present as a ribonucleoprotein particle cosedimenting in this region. Previous experiments had indicated that in actively dividing myoblasts 26 S RNA has a relatively short half-life but that it becomes “stable” after the cessation of mitosis just prior to fusion. RNA chase experiments performed in the present study show that the “short-lived” 26 S RNA from dividing myoblasts, which is present as a ribonucleoprotein particle, does not enter the heavy polysomes. In contrast, the more stable 26 S RNA also initially present as a ribonucleoprotein, just prior to and in the early stages of fusion, can be shown by chase experiments to enter the heavy polysomes later in fusion. Hence accumulation of 26 S RNA seems to precede its activation as a messenger.