Cell cycle regulation of the cyclin A, cdc25C and cdc2 genes is based on a common mechanism of transcriptional repression

Abstract

The S/G2-specific transcription of the human cdc25C gene is due to the periodic occupation of a repressor element ('cell cycle-dependent element'; CDE) located in the region of the basal promoter. Protein binding to the major groove of the CDE in G0 and G1 results in a phase-specific repression of activated transcription. We now show that CDE-mediated repression is also the major principle underlying the periodic transcription of the human cyclin A and cdc2 genes. A single point mutation within the CDE results in a 10- to 20-fold deregulation in G0 and an almost complete loss of cell cycle regulation of all three genes. In addition, the cdc25C, cyclin A and cdc2 genes share an identical 5 bp region ('cell cycle genes homology region'; CHR) starting at an identical position, six nucleotides 3' to the CDE. Strikingly, mutation of the CHR region in each of-the three promoters produces the same phenotype as the mutation of the CDE, i.e. a dramatic deregulation in Go. In agreement with these results, in vivo DMS footprinting showed the periodic occupation of the cyclin A CDE in the major groove, and of the CHR in the minor groove. Finally, all three genes bear conspicuous similarities in their upstream activating sequences (UAS). This applies in particular to the presence of NF-Y and Sp1 binding sites which, in the cdc25C gene, have been shown to be the targets of repression through the CDE. Our results strongly suggest that the CDE/CHR-mediated repression of activation by a specific set of transcription factors is a common mechanism of cell cycle-regulated transcription of S/G2-specific genes.