CYCLIN-A EXPRESSION IS UNDER NEGATIVE TRANSCRIPTIONAL CONTROL DURING THE CELL-CYCLE

Transcription of the gene coding for cyclin A, a protein required for S-phase transit, is cell cycle regulated and is restricted to prolifer ating cells. To further explore transcriptional regulation linked to c ell division cycle control, a genomic clone containing 5' flanking seq uences of the murine cyclin A gene was isolated. When it was fused to a luciferase reporter gene, it was shown to function as a proliferatio n-regulated promoter in NIH 3T3 cells. Transcription of the mouse cycl in A gene is negatively regulated by arrest of cell proliferation. A m utation of a CC-rich sequence conserved between mice and humans is suf ficient to relieve transcriptional repression, resulting in a promoter with constitutively high activity. In agreement with this result in v ivo footprinting reveals a protection of the cell cycle-responsive ele ment in G(0)/early G(1) cells which is not observed at later stages of the cell cycle. Moreover, the footprint is present in dimethyl sulfox ide-induced differentiating and not in proliferating Friend erythroleu kemia cells. Conversely, two other sites, which in vitro bind ATF-1 an d NF-Y, respectively, are constitutively occupied throughout cell cycl e progression.