Cell Cycle‐dependent Modulation of Promoter Activities of RB and WAF1/Cip1 Genes

A universal inhibitor of cyclin‐dependent kinases, WAF1/Cip1 can dephosphorylate the RB gene product to arrest the cell cycle at the G1 phase. Here we show that the mRNA level and the promoter activities of the RB and WAF1/Cip1 genes exhibit cell cycle‐dependent change when cells are released from either serum‐starvation or the confluent cell state with serum. RB expression and promoter activity are elevated at middle to late G1. In contrast, the mRNA and promoter activity of the WAF1/Cip1 gene increase at early G1. These results suggest that the RB and WAF1/Cip1 expression and promoter activities depend not only on serum, but also on the cell cycle progression itself. Moreover, we identified the responsive region for serum‐released cell cycle progression in the RB promoter and mapped it to the region between –4 and –182 relative to the initiating codon of the RB gene. The region in the WAF1/Cip1 promoter responsible for the serum‐released cell cycle progression mapped not to the p53 binding site, but to the 374 base‐pair region between –1770 and –1396 from the transcription start site.