Myogenic regulatory factors can activate TATA-containing promoter elements via an E-box independent mechanism

We have studied the effect of several myogenic regulatory factors on the activity of the promoter for a mouse gene encoding a skeletal myosin heavy chain (MyHC) expressed in adult (type IIB) muscle fibers. Co-transfection of myogenic factors is necessary for activity of the IIB promoter in mouse C2 myotubes in culture but not in quail myotubes in culture. Although this promoter contains one E-box within the first 192 base pairs upstream of the transcriptional start site, mutations in this motif demonstrate that it is not required for the transactivation effect of the myogenic factors. Analysis of other mutants suggests that the MEF2 and MHox DNA-binding factor binds to an evolutionarily conserved AT-rich motif. In addition, the IIB promoter appears to require the conserved TATA motif (CTATAAAAG) in order to be activated by the AT-rich sequences. The IIB promoter constructs produce RNA transcripts which begin at the natural site of transcriptional initiation in quail myotubes and in mouse C2 myotubes after co-transfection with myogenic factors; a second, minor, start site is also used in the co-transfected C2 myotubes. Results obtained after transfection of the mouse IIB promoter constructs in quail myotube cultures suggest that the overexpression of myogenic factors in C2 cultures does not result in an environment in which the control of IIB MyHC promoter activity is aberrant. Therefore, either the myogenic factors themselves, or other proteins induced by them, seem to interact directly with the basal transcription seem to interact directly with the basal transcription machinery to allow muscle-specific gene expression.