The E1 and E2F sites are functionally important to the activity of the Cdc6 promoter. (A) A schematic representation of three luciferase (Luc) reporter constructs, empty vector (pGL3-basic), and a construct driven by a wild-type (wt) Cdc6 promoter (Cdc6-wt) or a Cdc6 promoter harboring mutations within the E1 site (Cdc6-mut1). (B) C3H10T1/2 cells were separately transfected with empty vector and with Cdc6-wt or Cdc6-mut1, either alone or with increasing amounts of a plasmid encoding MyoD (pCMV-MyoD). Activity is expressed as fold increase in comparison with activity observed with empty vector after correcting for transfection efficiency (see Materials and methods). (C) Transcriptional-activity of the empty vector or the Cdc6-luciferase constructs in C2C12 myoblasts with or without mutations within E1, E2F, or both sites. (B and C) Error bars represent SEM.