AN IN-VIVO ANALYSIS OF TRANSCRIPTIONAL ELEMENTS IN THE MOUSE ALPHA-MYOSIN HEAVY-CHAIN GENE PROMOTER

During development in the murine ventricle, there is a switch in myosi n heavy chain gene (MyHC) transcription. The beta-MyHC is expressed in the ventricles during foetal development, but is shut down at or arou nd birth, at which time alpha-MyHC transcription is activated. This an tithetical switch is thought to be mediated by circulating levels of t hyroid hormone (TH) and both low and high affinity thyroid response el ements (TREs) have been identified in the proximal promoter region of the murine alpha-MyHC. Myosin gene expression in the atria is relative ly unaffected by the TH status. Previously, we used site-directed muta genesis of the promoter in a transgenic analysis to define those eleme nts responsible for high levels of transcription in vivo. These analys es focused on the role(s) of two cis elements, TRE(1) and TRE(2) that are located at -129 to -149 and -102 to -120, respectively, on the alp ha-MyHC promoter. Although the elements' ablation had differential eff ects on transgene expression, neither single mutation abolished transg ene expression completely. Here, we show that mutating both elements r esults in a complete inactivation of the transgene in both ventricles and atria under euthyroid conditions. However, expression still can be detected in the hyperthyroid state, implying that, although the TRE(1 ) and TRE(2) elements are critical elements for high levels of alpha-M yHC transcription in vivo, other promoter sites can mediate at least s ome degree of transcriptional activation.