FIGURE 4.

HP1α and HP1β, but not HP1γ, repress MyoD-mediated transcription. Cotransfection into HEK 293 cells of an MCK promoter-driven luciferase reporter plasmid with a fixed amount of MyoD expression vector (800 ng), and increasing amounts of pcDNA3 vector expressing HP1α (A), HP1β (B), or HP1γ (C). The quantities of different pcDNA3-HP1 expression vectors used were: 0, 100, 200, 300, and 500 ng. The total amount of pcDNA3 was normalized when necessary to 500 ng. Expression of MyoD and different HP1s was analyzed by Western blotting (data not shown). The inhibitory effect of HP1α and HP1β was specific and MyoD-dependent (compare black bars to gray bars). Indeed, it was not seen with β-galactosidase expression under a cytomegalovirus promoter (used as a normalization control for the transfection), nor with the MCK-luciferase vector in the absence of MyoD (gray bars). Black bars correspond to transfections with pEMSV-MyoD vector, and gray bars correspond to the empty vector control pEMSV.