The human embryonic myosin heavy chain. Complete primary structure reveals evolutionary relationships with other developmental isoforms.

We have isolated a single 6021-nucleotide cDNA fragment encoding the full length of the myosin heavy chain (MHC) isoform initially expressed in developing human limb muscle. The corresponding transcript is expressed in fetal, but not adult, human muscle, and the corresponding gene maps to human chromosome 17. Comparison of the full length nucleotide sequence with that of the orthologous rat gene transcript reveals 74, 90, and 80% similarities in the 5'-untranslated, coding, and 3'-untranslated regions, respectively. To precisely quantitate the degree of nucleotide sequence divergence between the human embryonic and other developmentally regulated MHC gene transcripts, we utilize the algorithm of Perler et al. (Perler, F., Efstratiadis, A., Lomedico, P., Gilbert, W., Kolodner, R. & Dodgson, J. (1980) Cell 20, 555-566) and make use of the codon-for-codon register attainable in alignments of the MHC rod encoding cDNA fragments. The results allow reconstruction of the order and relative timing of certain gene duplication events involved in the evolution of the multimembered mammalian MHC loci. By this analysis, the principal sarcomeric MHC gene expressed in the 14-day chick embryo is shown to be more distantly related to the mammalian embryonic MHC genes than to those expressed peri- and postnatally. Attention is focused on regional patterns of MHC sequence conservation, ordered with reference to the topology of our phylogenetic tree. We present a composite map depicting the deduced evolutionary age of various primary structural subdomains of the human embryonic MHC.