Cyclin A is a pivotal regulatory protein which, in mammalian cells, is involved in the S phase of the cell cycle. Transcription of the human cyclin A gene is cell cycle regulated through tight control of its promoter. We have previously shown that the ATF/CREB site, present in the cyclin A promoter, mediates transcriptional regulation by cAMP responsive element binding proteins. The main goal of the present study was to investigate whether this site is involved in transcriptional regulation of the gene. We have constructed stable NIH-3T3 cell lines that express the luciferase reporter gene under the control of normal or mutated versions of the cyclin A promoter. We show that the ATF/CREB is required to achieve maximal levels of transcription from the cyclin A promoter starting in late G1. We also show that down-regulation of the cyclin A promoter by p53 does not implicate a direct binding of p53 to its cognate consensus sequence but occurs probably by interference with trans-activating factors. This result suggests that p53 can interfere with transcription of the cyclin A gene, in the absence of a TATA sequence in the promoter.