p53 as an unstable protein in vitro likely requires stabilizing factors to act as a tumor suppressor in vivo. Here, we show that in human cells transfected with wild-type (WT) p53, Hsp90 and Hsp70 molecular chaperones maintain the p53 native conformation under heat-shock conditions (42 degrees C) as well as assist p53 refolding at 37 degrees C, during the recovery from heat shock. We also show that the interaction of WT p53 with WAF1 promoter in cells is sensitive to Hsp70 and Hsp90 inhibition already at 37 degrees C and further decreased on heat shock. The influence of chaperones on p53 binding to the WAF1 promoter sequence has been confirmed in vitro, using highly purified proteins. Hsp90 stabilizes the binding of p53 to the promoter sequence at 37 degrees C, whereas under heat-shock conditions the requirement for the Hsp70-Hsp40 system and its cooperation with Hsp90 increases. Hop co-chaperone additionally stimulates these reactions. Interestingly, the combined Hsp90 and Hsp70-Hsp40 allow for a limited in vitro restoration of the DNA-binding activity by the p53 oncogenic variant R249S and affect its conformation in cells. Our results indicate for the first time that, especially under stress conditions, not only Hsp90 but also Hsp70 is required for the chaperoning of WT and R249S p53.