Cloning of a trout fast skeletal myosin heavy chain expressed both in embryo and adult muscles and in myotubes neoformed in vitro

In fish, little is known about the isoforms of myosin heavy chain in developing muscle. Two cDNA libraries from whole skeletal muscle of embryo (eyed stage) (A) and from white muscle of 300 g body weight immature trout (B) were constructed. Three cDNA clones were isolated and characterised as encoding for a fast skeletal myosin heavy chain. Two cDNA clones A1 (1534 bp) and B6 (2203 bp) which were extracted from the two different libraries had the same nucleotide sequence including the 3' untranslated region. The third cDNA B8 (1606 bp) shared 98% identity with the others. The latter could possibly be an allelic isoform of the B6. Northern blot analysis revealed that the fast skeletal MyoHC transcripts were expressed throughout development from myotube appearance to the white muscle present at older stages (adult). These results suggest that this myosin heavy chain is present throughout muscle development in fish and are consistent with the hyperplastic growth of fish muscle. The amino acid sequence of the trout myosin heavy chain diverged from its mammalian and avian counterpart with respect to a higher level of glycine which could be related to an environmental adaptation by increasing thermal instability of the molecule.