Cell-cycle regulation of human B-myb transcription

We demonstrate here that activity of the human B-myb promoter is regulated during the cell cycle by the E2 transcription factor (E2F). Comparison of the human B-myb promoter sequence with that of its murine counterpart revealed an evolutionally conserved sequence that contains an E2F-binding site. In transiently transfected murine NIH3T3 and human HaCaT cells, luciferase (Luc) reporter activity directed by the human B-myb promoter was found to increase significantly in late G1/S phase of the cell cycle. Mutation of the promoter E2F site resulted in significantly greater Luc activity in NIH3T3 and HaCaT cells made quiescent by serum deprivation, indicating that E2F repressed transcription of this gene during G0. Analysis of E2F DNA-binding activity in G0 HaCaT cells revealed a distinct complex that apparently contained neither the retinoblastoma gene protein, pRb, nor the related p107 protein. De-repression of transcription in S phase was accompanied by the disappearance of this G0 E2F complex and the appearance of a distinct complex containing p107. In addition, complexes containing pRb were detected at both stages of the cell cycle.