DNA-dependent RNA polymerases in skeletal muscle cells differentiating in vitro

The main classes of RNA polymerase in cultured chick embryo skeletal muscle cells were identified and their levels determined during differentiation in vitro. Cell cultures contained a high proportion of muscle cells (>80%) and exhibited a high degree of fusion (>80%). The levels of RNA polymerases I and II increased by 1.8 and 1.5 times, respectively, from prefusion myoblasts (24 hr) to postfusion myotubes (120 hr). RNA polymerase III, a single peak on DEAE-Sephadex chromatography, was less than 7% of the total activity. The results suggest that the rates of synthesis of the main RNA types, which reportedly decline during myogenesis, are not determined by the relative levels of the main RNA polymerases and are more likely a reflection of chromatin template availability.