TATA-box dependent trans-activation of the human HSP70 promoter by Myb proteins

Abstract

The oncogene v-myb and its cellular progenitor c-myb encode nuclear, DNA-binding phosphoproteins that function as transcriptional regulators. Previous studies have demonstrated the existence of two distinct transactivation mechanisms for Myb-inducible promoters. One of these mechanisms, exemplified by the chicken mim-1 promoter, involves specific binding of Myb protein to Myb binding sites located upstream of the transcriptional start site. A second mechanism, exemplified by the human HSP70 promoter, does not require the presence of specific Myb binding sites. Here, we have more closely investigated transactivation of the human HSP70 promoter by Myb proteins. Mutational analysis of the human HSP70 promoter failed to show a correlation between Myb-inducibility and the presence of a particular upstream transcription factor binding site, but instead showed that Myb-inducibility was dependent on the presence of a particular type of TATA-box. This suggests that the TATA-box is involved in transactivation by v-Myb and that only a subset of TATA-boxes confers Myb-inducibility. Activation of the HSP70 promoter is dependent on the presence of most of the carboxy terminal amino acid sequences of v-Myb, including the previously identified acidic transactivation domain and a putative leucine-zipper region. Finally, we show that the HSP70 promoter is activated in a Myb binding site independent fashion not only by v-Myb, but also by chicken c-Myb and chicken B-Myb. These observations raise the possibility that the ability to transactivate in a Myb binding-site independent manner is a common property among Myb proteins. Since chicken B-Myb has been shown previously to be unable to transactivate a Myb binding-site containing promoter, such as the mim-1 promoter (Foos et al., 1992), our results also show that both modes of transactivation are independent of each other.