Regulation of U3 snRNA expression during myoblast differentiation.

Differentiation of proliferating rat L6 myoblasts to syncytial multinucleated myotubes results in a significant downshift in the rate of U3 snRNA gene transcription, paralleling the decrease in rRNA synthesis previously documented. Coordinate production of U3 snRNA and rRNA during the differentiation process adds further support for a role of U3 snRNA in ribosome biogenesis. Despite the dramatic decrease in U3 snRNA transcription during differentiation, a corresponding drop in the cellular level of U3 snRNA does not occur. In myotubes, the amount of U3 snRNA is regulated at the post-transcriptional level in which there is a significant accumulation of U3 snRNA in the cytoplasm of myotubes. This intracellular redistribution of U3 snRNA may significantly affect the entire process of rRNA maturation or result from the decrease in ribosome production accompanying terminal differentiation of myoblasts.