Abstract:In this study,two types of myosin heavy chain isoform gene were isolated from fast skeletal muscles of silver carp(Hypophthalmichthys molitrix) acclimatized in winter and summer by RT-PCR and 3’-RACE,and named low temperature-type(sc-w) and high temperature-type(sc-s),respectively.These two isoform genes encompassed parts of 3′-translated and entire 3′-untranslated regions and encoded amino acid sequences of the C-terminal for light meromyosin(LMM).The marked differences between sc-w and sc-s were observed in the upstream and downstream regions of the stop codon.The nucleotide and deduced amino acid sequences of these two isoforms showed 79.5% and 88.1% identities,respectively.Isoform-specific differences were clearly observed between sc-w and sc-s in 15 amino acid residues.Furthermore,among these amino acid mutations,11 mutations occurred at the highly conserved residue sites of LMM heavy chain from other freshwater fish belonging to Cyprinidae except for the grass carp 10 C type(gc10).These results seem to suggest that myosin heavy chain isoforms expressed in low temperature-acclimated freshwater fish require some amino acid substitutions to adapt its assembly characteristics to different environmental temperature,and subsequently reveal functional properties different from those expressed in high temperature-acclimated freshwater fish at protein level.On the other hand,phylogenetic analysis further indicated that the myosin heavy chain isoforms from fish inhabiting similar environmental temperatures have close gene distances,demonstrating the effects of temperature on genomic divergence and functional evolution of myosin heavy chains from eurythermal freshwater fish.
