PERIODIC CDC25C TRANSCRIPTION IS MEDIATED BY A NOVEL CELL-CYCLE-REGULATED REPRESSOR ELEMENT (CDE)

We show that the cell cycle-regulated transcription of the TATA-less c dc25C gene in late S/G(2) is largely mediated by a novel promoter elem ent (CDE) located directly 5' to one of the two major transcription in itiation sites. Genomic dimethylsulfate footprinting experiments, usin g either synchronized or sorted normally cycling cells, show the forma tion in vivo of a CDE-protein complex in both G(0) and G(1) cells and its dissociation in G(2). Mutation of the CDE severely impairs cell cy cle regulation of the cdc25C promoter and results in high expression i n G(0)/G(1), indicating that the CDE functions as a cell cycle-regulat ed cis-acting repressor element. Cell cycle regulation is also lost up on removal of the enhancer region located immediately upstream of the CDE, but is largely restored when this enhancerless minimal cdc25C pro moter fragment is linked to the constitutive SV40 early enhancer. This indicates that the CDE is dependent on the presence of a transcriptio nal enhancer to effect cell cycle regulation. Our observations suggest that the periodic activation of the cdc25C gene in late S/G(2) is bro ught about, at least in part, by a unique regulatory mechanism involvi ng the cell cycle-regulated dissociation of a repressor from the CDE.