The two myosin isoenzymes of chicken anterior latissimus dorsi muscle contain different myosin heavy chains encoded by separate mRNAs.

The two myosin isozymes (SM1 and SM2) of the anterior latissimus dorsi muscle of the chicken change in relative concentration during development. As SM1 decreases from 13 days of embryonic growth through 1 year of adult maturation, SM2 increases. In the adult muscle SM2 accounts for over 95% of the total myosin. The myosin heavy chains of the two isozymes are distinctly different and may be separated from each other by 5% SDS polyacrylamide gel electrophoresis. The faster migrating myosin heavy chain is identified as originating from SM1 and the slower migrating myosin heavy chain from SM2 myosin isozymes. The myosin heavy chains change in relative concentration during development exactly parallel with changes in SM1 and SM2 isozyme levels. Peptide map analysis also reveals that SM1 myosin heavy chains and SM2 myosin heavy chains are distinctly different. When RNA from the ALD muscle is added to reticulocyte lysate protein synthesizing systems the translation products are shown to include both SM1 and SM2 myosin heavy chains. These comigrate exactly on 5% SDS polyacrylamide gels with authentic counterparts from ALD muscle. Finally, when peptide maps of SM1 and SM2 myosin heavy chains synthesized in the reticulocyte lysate are compared they are again found to be distinctly different and each is identical to a peptide map of respective authentic SM1 and SM2 myosin heavy chains. It is concluded that the myosin heavy chains of SM1 and SM2 myosin isozymes of ALD muscle have different primary structures and that they are encoded by two distinctly different mRNAs.