REGULATION OF CELL GROWTH-DEPENDENT EXPRESSION OF MAMMALIAN CDC6 GENEBY THE CELL-CYCLE TRANSCRIPTION FACTOR E2F

CDC6 of Saccharomyces cerevisiae regulates the DNA replication initiat ion through the origin recognition complex (ORC), Identification of a human homolog of the CDC6 gene (HsCdc6) suggests a universal role of t he gene product in DNA replication. Expression of HsCdc6 is growth-reg ulated. We investigated the molecular basis of growth-regulated expres sion of mammalian Cdc6. The promoter activity of isolated HsCdc6 upstr eam region was activated at late G1 and G1/S boundary in the cell cycl e of rat embryonic fibroblast REF52 cells by the addition of serum. Th e isolated promoter was activated by exogenous expression of E2F witho ut serum stimulation. However a mutant promoter lacking the E2F recogn ition sites failed to respond to serum stimulation and exogenous expre ssion of E2F, Expression of endogenous Cdc6 was induced by exogenous e xpression of E2F, Therefore, we concluded that the growth-regulated ex pression of mammalian Cdc6 was mediated by E2F, Moreover, we demonstra ted that exogenous overexpression of either HsCdc6 or HsOrc1 failed to induce DNA synthesis unlike overexpression of E2F1, even though E2F1 induced both Cdc6 and Orc1, suggesting that E2F may regulate the expre ssion of another gene(s), besides Cdc6 and Orc1, required for inductio n of cellular DNA synthesis in mammalian cells.