Expression of myosin heavy chain transcripts during Xenopus laevis development.

In amphibians, the composition and pattern of expression of the myosin heavy chain (myHC) gene family are mostly unknown. To understand better the regulation of this complex family, we screened cDNA libraries from swimming tadpole and adult leg muscle RNA and have isolated and partially sequenced clones for several myHCs. Two of these, E3 and E19, first appear in mesoderm at late gastrula stage and are coexpressed with muscle actin. They increase in abundance in trunk and tail myotome until metamorphosis, when they begin to decline. E3 and E19 are also both expressed in hind leg muscle at the beginning of metamorphosis, but decline to low levels in adult leg muscle. A new transcript, A7, first appears during early metamorphosis in both the tail and hind leg skeletal muscle. A7 transcripts then decline in degenerating tail but persist in hind leg through metamorphosis and in adults. Two other embryonic myHCs, E14 and E15, code for isoforms closely related to E19 and may be alleles or duplications. Although sequence comparisons with myHCs from other vertebrates could not correlate the transcripts with specific protein isoforms, from the pattern of expression E3 and E19 apparently code for embryonic fast skeletal myHC isoforms, whereas A7 codes for an adult skeletal isoform.