Unprocessed myogenin transcripts accumulate during mouse embryogenesis.

The gene myogenin encodes a helix-loop-helix protein whose function is critical to the integrity of the mammalian myogenic cascade. In vitro, the expression of this gene immediately precedes terminal differentiation, as measured by the synthesis of those proteins that make up the contractile apparatus. However, during mammalian development, expression of myogenin and the appearance of sarcomeres are separated by at least 1 week. The observation that early embryos (10.5 days post coitum) do not possess myogenin protein despite the fact that transcripts are detected (Cusella-De Angelis et al. (1992) J. Cell Biol. 116, 1243-1255), suggests the possibility of post-transcriptional regulation. Using polymerase chain reaction and in situ analyses, we report here that the mouse embryo accumulates a significant pool of unprocessed myogenin RNA in the developing somites (10.5 days post coitum). These results indicate that post-transcriptional regulation of myogenin may occur at the RNA processing level.