A novel human myosin alkali light chain is developmentally regulated. Expression in fetal cardiac and skeletal muscle and in adult atria.

We have isolated cDNA recombinant phages encoding the embryonic isoform of the myosin alkali light chain (MLC1emb) from a human fetal skeletal muscle library. The cDNA clones were detected by their weak cross-hybridization to a human MLC1F and MLC3F cDNA clone. Nucleotide sequence analysis of the complete cDNA (GT14) revealed an open reading frame for 197 amino acids. The derived protein sequence constitutes the first structural information on this myosin isoform of any organism. Remarkable structural similarities to other alkali MLC polypeptides, particularly to those of the slow-muscle type, are evident. Under conditions of high stringency, the GT14 clone hybridized to an abundant mRNA species in fetal ventricular muscle and adult atrial muscle, whereas in fetal skeletal muscle only a very weakly hybridizing mRNA component was detected. These mRNAs were indistinguishable by size and the thermal stability of their hybrids formed with the DNA insert of clone GT14. We therefore conclude that identical mRNA is expressed in these tissues, presumably transcribed from the same gene. According to its pattern of mRNA expression, the novel MLC isoform described here was designated as "embryonic and atrial myosin light chain" (MLC1emb/A) in reference to its developmental stage-specific and tissue-specific appearance in embryonic skeletal muscle, fetal ventricle and adult atrium.