FIG. 8.
Activation of the c-myb promoter by E2Fmyb-sp and/or SP-1 through the E2F element during the G1 phase of the cell cycle. (A) The indicated myb-Luc reporter plasmids containing wild-type or mutant E2F elements were cotransfected with pCMV-βgal into NIH 3T3 cells. The cells were placed in low (0.5%) serum 4 h after the removal of the calcium phosphate precipitates. The cells remained in low serum for >48 h to induce quiescence, at which point serum was added (time zero). At the indicated time points, cells were removed for cell cycle analysis by flow cytometry and for determination of luciferase and β-galactosidase activities. (B) An E2F1 luciferase reporter plasmid containing wild-type E2F elements was analyzed in parallel as a control. Luciferase values from a representative experiment (normalized for β-galactosidase activity) (A and B) and percentages of cells in each phase of the cell cycle at the indicated time points (C) are shown.