We have investigated the effects of deregu-

Cyclin E controls progression through the G 1 phase of the cell cycle in mammalian fibroblasts and potentially in many other cell types. Cyclin E is a rate-limiting activator of cdk2 kinase in late G 1 . The abundance of cyclin E is controlled by phase-specific fluctuations in the mRNA level; in mammalian fibroblasts, mRNA is not detected under conditions of serum starvation and is accumulated upon serum stimulation, with expression starting in mid-G 1 . Here, we report the cloning of the murine cyclin E promoter. We isolated a 3.8-kb genomic fragment that contains several transcriptional start sites and confers cell cycle regulation on a luciferase reporter gene. This fragment also supports transcriptional activation by adenovirus E1A, a known upstream regulator of cyclin E gene expression. An E2F binding site which is required for G 1 -specific activation of the cyclin E promoter in synchronized NIH 3T3 cells was identified in this fragment.Regulation of cell cycle progression in mammalian cells depends on the sequential activation of a series of cyclin-dependent kinases. The catalytic activity of a given cyclin-dependent kinase subunit depends on its correct posttranslational modification and on its association with the appropriate cyclin, which acts as a regulatory subunit (reviewed in reference 28). Thus, phase-specific fluctuations in the abundance of various cyclins constitute one mechanism by which the timing of cyclindependent kinase activation is controlled. It was shown that phase-specific transcriptional activation of cellular genes contributes to the control of cell cycle progression; in particular, heterodimeric transcription factor E2F/DP (referred to as E2F [17]) mediates the activation of several cellular genes at the G 1 -to-S transition (27). More recently, it was shown that E2F binding sites determine activation at the G 1 -to-S transition of the genes encoding E2F-1 (9, 11), a subunit of the E2F/DP heterodimer, and cyclin A (26), a key regulator of the G 1 -to-S transition (24, 33), indicating a regulatory link between the ability of a cell to enter S phase and the appropriate activation of E2F during G 1 .Cyclin E (13, 18) is expressed in mid-G 1 ; it associates with and activates cdk2 kinase (6,14). Conditional overexpression of cyclin E results in a decrease in the length of the G 1 interval, consistent with an acceleration of the G 1 -to-S transition (25, 31). Microinjection of anti-cyclin E antibodies during G 1 inhibits entry into S phase in normal human fibroblasts (22), and genetic evidence suggests that cyclin E controls S phase entry during early embryogenesis in Drosophila melanogaster (12). These findings strongly suggest that cyclin E and its associated kinase are centrally involved in the decision to enter S phase in most if not all eukaryotic cells. Cyclin E mRNA is absent from serum-starved normal fibroblasts and appears in mid-G 1 (13). Regulation of cyclin E gene expression appears to be involved in the transformation of mammalian cells by viral oncogenes, since a rapid ind...

In this study, we characterized the 5h regulatory region of the murine cyclin E gene and analysed activation of the gene by the E7 oncogene of human papillomavirus type 16 in transfection experiments. We found that the murine cyclin E promoter is composed of multiple regulatory elements, and we present evidence for at least two independent transcription units, designated P1 and P2. Overlapping binding sites for the cellular transcription factors Sp1 and E2F were identified in both promoters, and we found that E2F-mediated activation of transcription is inhibited by Sp1 in cotransfection experiments. The E2F/Sp1 binding sites contribute to transcriptional activation by E7, and the data suggest that the cyclin E gene is rendered E7-inducible through the combination of several cis-acting elements which display only weak intrinsic responsiveness to E7.