Synthesis of myosin light chains and accumulation of translatable mRNA coding for light chain-like polypeptides in differentiating muscle cultures.

Rat skeletal muscle myosin contains small subunits (light chains, here designated LC, LC, LC) of molecular weight 23,000, 17,000 and 15,000, respectively. The synthesis of myosin light chains during differentiation and the accumulation of mRNA which codes for these proteins were investigated in differentiating rat skeletal muscle cultures. When cultures were labeled prior to cell fusion, radioactive light chains which co-migrated with skeletal muscle myosin light chains on gel electrophoresis were absent or only barely detectable. The low molecular weight peptides which were associated with the heavy chain of myosin extracted from pre-fusion cultures differed in their electrophoretic mobility from light chains of skeletal muscle myosin. Following cell fusion, the amount of labeled LC and LC increased rapidly. The synthesis of LC was barely detectable during cell fusion and never exceeded one-fifth of the amounts of LC and LC synthesized. Polyadenylated RNA extracted at different times during differentiation was translated in the wheat germ cell-free system. The products were analyzed on isoelectric focusing-SDS two-dimensional gel electrophoresis, and the radioactivity of the polypeptides co-migrating with myosin light chains was measured. Small amounts of radioactive products co-migrating with LC and LC became detectable among products of RNA preparations extracted several hours prior to cell fusion. However, the cell-free system directed by post-fusion RNA synthesized much larger amounts of LC- and LC-like polypeptides. Rapid accumulation of translatable mRNA for LC and LC was closely correlated with cell fusion. Radioactive polypeptides co-migrating with LC were synthesized in significant amounts in a cell-free system directed by pre-fusion RNA and increased only moderately when RNA extracted after fusion was used.