MULTIPLE DNA ELEMENTS ARE REQUIRED FOR THE GROWTH-REGULATION OF THE MOUSE E2F1 PROMOTER

To prepare for the DNA synthesis (S) phase of the cell cycle, transcri ption of many genes required for nucleotide biosynthesis increases. Th e promoters of several of these genes contain binding sites for the E2 F family of transcription factors, and, in many cases, mutation of the se sites abolishes growth-regulated transcription. The RNA levels of o ne family member, E2F1, increase about 15-fold at the G(1)/S-phase bou ndary and expression of E2F1 in quiescent cells activates transcriptio n from some G(1)/S-phase-specific promoters, suggesting that E2F1 play s a critical role in preparing cells to enter S phase. To elucidate th e signal transduction pathway leading to the activation of genes requi red for DNA synthesis, we are investigating the mechanism by which exp ression of E2F1 is regulated. To determine whether levels of E2F1 mRNA are controlled by changes in promoter activity, we have cloned and ch aracterized the mouse E2F1 promoter. Sequence analysis revealed two se ts of overlapping E2F-binding sites located between -12 and -40 relati ve to the transcription initiation site. We show that these sites bind cellular E2F and that an E2F1 promoter fragment can be activated up t o 100-fold by coexpression of E2F proteins. We also show that the acti vity of this E2F1 promoter fragment increases similar to 80-fold at th e G(1)/S-phase boundary and that this activation is, in part, regulate d by G(0)-specific repression via the E2F sites. However, the E2F site s are not sufficient to mediate growth-regulated transcriptional activ ity; our results indicate that multiple DNA elements are required for transcription regulation of the E2F1 promoter at the G(1)/S-phase boun dary.