Fig. 5. Tax represses myoD enhancer activity and prevents MyoD from stimulating its enhancer. A, C2.7 cells were transfected with 0.5 µg of the pEnh-myoD-LacZ plasmid alone or together with 1 µg of pLTR-Tax. Twenty-four h after transfection, the cells were either cultured for 24 h in proliferation medium (PM) or switched to differentiation medium (DM) and further cultured for 48 h. They were harvested and assayed for ß-galactosidase activity, as indicated in "Materials and Methods." The enzyme activities are the means of three experiments (bars; SD). Results are expressed as fold activation of the ß-galactosidase activity in cells transfected with the pEnh-myoD-LacZ plasmid cultured in proliferation medium. B, C3H10T1/2 cells were transfected with 0.5 µg of the pEnh-myoD-LacZ plasmid, alone or together with 1 µg of pLTR-Tax and/or 1 µg of pCMV-MyoD. Forty-eight h after transfection, cells were harvested and assayed for ß-galactosidase activity, as indicated in "Materials and Methods." The enzyme activities are the means of three experiments (bars; SD). Results are expressed as fold activation of the ß-galactosidase activity in cells transfected with the pEnh-myoD-LacZ plasmid. , PM; {blacksquare}, DM.