C-MYB-INDUCED TRANSACTIVATION MEDIATED BY HEAT-SHOCK ELEMENTS WITHOUTSEQUENCE-SPECIFIC DNA-BINDING OF C-MYB

The c-myb proto-oncogene product (c-Myb) can transactivate the human h sp70 promoter in a transient cotransfection assay. The present studies have demonstrated that the heat shock element (HSE) in the hsp7O prom oter mediates trans-activation by c-Myb. Mutagenesis of the DNA sequen ce in HSE indicated that the NGAAN motif is necessary for not only the heat shock response but also the c-Myb-induced trans-activation. The HSE in the hsp70 promoter does not contain a c-Myb-binding site, imply ing that the sequence-specific DNA binding of c-Myb is not required fo r the HSE-dependent trans-activation by c-Myb. We had demonstrated tha t a disruption of the leucine zipper motif in the central portion of t he c-Myb molecule increased the degree of c-Myb-induced trans-activati on of the promoter containing c-Myb-binding sites, suggesting that a p utative inhibitor binds to c-Myb through this leucine zipper (Kanie-Is hii, C., MacMillan, E. M., Nomura, T., Sarai, A., Ramsay, R. G., Aimot o, S., Ishii, S., and Gonda, T J. (1992) Proc. Natl. Acad. Sci. U. S. A. 89, 3088-3092). However, disruption of the leucine zipper in c-Myb abolished the HSE-dependent trans-activation by c-Myb, whereas deletio n of the transcriptional activation domain containing acidic amino aci ds in c-Myb did not abolish the HSE-dependent trans-activation by c-My b. These results suggest that c-Myb can activate transcription of the gene containing HSEs by interacting with unidentified trans-acting fac tor(s) but not by a direct binding to the promoter through its DNA-bin ding domain.