FIG. 4.
p53 downregulates CHK1 RNA levels. (A) Exponentially growing HCT116 (p53+/+) and their derivative null cells (p53−/−) were treated with daunorubicin (Dauno, 0.22 μM) or actinomycin D (Act. D, 5 nM), and total RNA samples were prepared at the indicated time points. Northern blots were performed using the indicated 32P-labeled plasmid probes. The GAPDH plasmid probe was used as a normalizing control. (B) Quantitative representation of the experiment reported in panel A. For CHK1, the initial mRNA levels were taken as 1. For p21, the maximal p21 mRNA levels were also taken as 1. (C) Exponentially growing H1299 expressing inducible wild-type p53 (WT) or p53 [22–23] mutant (22–23) cells were extensively washed to remove tetracycline. After the indicated hours, total RNA samples were prepared and Northern blots were performed using the indicated 32P-labeled plasmid probes. The GAPDH plasmid probe was used as a normalizing control.