TYPE-IIX MYOSIN HEAVY-CHAIN TRANSCRIPTS ARE EXPRESSED IN TYPE IIB FIBERS OF HUMAN SKELETAL-MUSCLE

Several members of the sarcomeric myosin heavy chain (MHC) gene family have been mapped in the human genome but many of them have not yet be en identified. In this study we report the identification of two human skeletal MHC genes as fast IIa and IIx MHC based on pattern of expres sion and sequence homology with the corresponding rat genes in the S'- translated and untranslated regions. The distribution of these two gen e products as well as that of the beta/slow MHC gene was analyzed in h uman skeletal muscles by in situ hybridization. The distribution of be ta/slow, IIa, and IIx MHC transcripts defines three major muscle fiber types expressing a single MHC mRNA, i.e., either beta/slow, IIa, or I Ix MHC mRNA, and two populations of hybrid fibers coexpressing beta/sl ow with IIa or IIa with IIx MHC mRNA. Fiber typing by ATPase histochem istry shows that IIa MHC transcripts are more abundant in histochemica l type IIa fibers, whereas IIx MHC transcripts are more abundant in hi stochemical type IIb fibers.