The same myosin alkali light chain gene is expressed in adult cardiac atria and in fetal skeletal muscle.

We have isolated from a cDNA library constructed using mouse cardiac mRNA sequences, a clone (pC6) homologous to part of the mRNA encoding the myosin alkali light chain MLC1A from adult mouse atria. This sequence also hybridizes to mRNA encoding the fetal light chain form MLC1emb expressed in both fused myotubes in culture and in 18 day fetal skeletal muscle. These mRNA sequences are indistinguishable from the MLC1A messenger both on the basis of size and of their thermal stability of hybridization. In vitro translation of mRNA selected by hybridization with pC6 results in a protein that comigrates with the fetal MLC1emb isoform, and two-dimensional gel electrophoresis of adult atrial and fetal skeletal muscle proteins shows MLC1A and MLC1emb to be indistinguishable in the mouse. Southern blot hybridization of clone pC6 to mouse genomic DNA and the analysis of restriction fragment length polymorphisms between different mouse species demonstrates the presence of a single hybridizing locus in the mouse genome. These data provide strong evidence that the atrial MLC1A and fetal skeletal MLC1emb isoform are encoded by the same gene and by the same mRNA and are thus identical proteins.