A GENE WITH HOMOLOGY TO MYOGENIN IS EXPRESSED IN DEVELOPING MYOTOMAL MUSCULATURE OF THE RAINBOW-TROUT AND IN-VITRO DURING THE CONVERSION OFMYOSATELLITE CELLS TO MYOTUBES

We report the cloning of a new trout myogenic cDNA which encodes helix -loop-helix protein homologous to the myogenic factor myogenin. Northe rn analyses indicate that trout myogenin (Tmyogenin) transcripts accum ulate in large amounts in the myotomal musculature of embryos and frys . In adults, transcripts concentrate within the thin lateral layer of red (slow oxydative) muscle fibres. They are present only in low amoun ts in white (fast glycolytic) muscle fibres which constitute the major part of the trunk musculature. Using an in vitro myogenesis system, w e observed that the trout myogenin encoding gene is not activated unti l myosatellite cells fuse to generate multinucleated myotubes, indicat ing that Tmyogenin lies downstream of muscle determination factors. Al l these observations show that in a major taxinomic group like teleost s, a gene with homology to myogenin exists. Its activation during myog enesis suggests that it acts as a major developmental regulator of mus cle differentiation.