Tropomyosin is an essential component of the sarcomeric thin filament in striated muscle that participates in the regulation of muscle contraction through Ca(2+)-mediated activation. The two predominant tropomyosin isoforms expressed in striated muscle are alpha- and beta-tropomyosin, which exhibit an 86% amino acid identity between themselves. Previous studies by our laboratory utilized a transgenic mouse system to overexpress beta-tropomyosin in the heart to address the functional differences between these two tropomyosin isoforms. Interestingly, when a high percentage of beta-tropomyosin replaces alpha-tropomyosin in the hearts of transgenic mice, the mice die due to severe cardiac abnormalities. In this study, we have rescued these high expression beta-tropomyosin mice by turning off the alpha-myosin heavy chain promoter, which is driving the beta-tropomyosin transgene. This down-regulation of the alpha-myosin heavy chain promoter was accomplished by the administration of 5-propyl-2-thiouracil, which disrupts thyroid hormone synthesis and inhibits promoter activity through thyroid regulatory elements located in the 5'-flanking region of the promoter. Results show that as beta-tropomyosin expression is down-regulated, alpha-tropomyosin expression is increased. Also, alpha- and beta-myosin heavy chain expression is modified in response to the changes in thyroid hormone expression. Morphological analysis of these rescued mice show a moderate pathological phenotype, characterized by atrial myocytolysis; echocardiographic analyses demonstrate altered ventricular functions, such as peak filling rates and left ventricular fractional shortening. This is the first report demonstrating that transcriptional regulatory elements located within the alpha-myosin heavy chain promoter can be manipulated to rescue potentially lethal phenotypes, such as high expression beta-tropomyosin transgenic mice.