FIG. 4.

p53 sequence-specific DNA-binding activity in cells expressing E1B 55-kDa protein. (A) Electrophoretic mobility shift assay of p53 DNA-binding activity. The nuclear extracts of cell lines G401, G401-CC3, and 293, which contained about equal amounts of p53 as shown on the top of panel A, were incubated with a radioactive oligonucleotide bearing consensus p53-binding sites (WT). Competing unlabeled WT oligonucleotide or mutant oligonucleotide (MUT) as well as monoclonal anti-p53 antibody PAb421 was added in some reactions as indicated. Raji nuclear extract was used as positive control. Specific p53-DNA complexes are denoted with arrowheads. (B) p21 levels in G401, G401-CC3, and 293 cells. The p53 levels in the total cell extracts of these three cell lines were estimated by Western blotting (top), and the p21 protein concentration in total cell extracts containing about equal amounts of p53 was determined by Western blotting using an anti-p21 polyclonal antibody (C-19; Santa Cruz Biotechnology).