Cardiac myosin heavy chain isozymic transitions during development and under pathological conditions are regulated at the level of mRNA availability.

We have shown that the level of two ventricular MHC mRNAs, fetal and adult, can account for the observed isozymic transitions of the myosin protein during normal development and under pathological conditions. Moreover, these MHC mRNAs are encoded by two genes that are linked in the genome, less than 5 kilobases apart and are organized according to their developmental expression. The fact that the two ventricular MHC genes are very closely related at the nucleotide sequence level, and yet respond in opposite direction to the same stimulus, makes these genes an excellent model in which to study the possible mechanisms involved in the gene switching.