Mutation of conserved domain II alters the sequence specificity of DNA binding by the p53 protein.

We have mutagenized human p53 expressed in yeast and selected two mutants, 121F and 123A, which activate transcription from one, rather than the normal two, copies of the consensus p53 DNA binding sequence. Both mutants have a 6-fold increase in affinity for a single copy of the sequence GGG CATG CCC. The 121F mutant has a decrease, and the 123A mutant an increase, in the affinity for the sequence GAA CATG TTC. This genetic and biochemical evidence supports the crystallographic finding that amino acid 120 contacts guanine in the major groove at the second position in the consensus. The major p53 binding site in the p21WAF1/CIP1 promoter resembles the GAA CATG TTC form of the consensus. Compared with wild type p53, the 121F mutant has a 7-fold lower affinity for the p21WAF1/CIP1 site in vitro, and the 121F mutant is defective in p21 induction in vivo. Mutants with subtly altered sequence specificity may facilitate dissection of downstream pathways activated by p53.