B-MYB AND CYCLIN D1 MEDIATE HEAT-SHOCK ELEMENT DEPENDENT ACTIVATION OF THE HUMAN HSP70 PROMOTER

Previous studies have shown that B-Myb, a conserved member of the Myb transcription factor family, is a potent activator of the promoter of the human HSP70 gene but does not activate promoters containing Myb bi nding sites. We have now investigated the transactivation properties o f B-Myb in more detail. We here report that B-Myb activates the HSP70 promoter by a novel mechanism which involves the heat shock element (H SE). Deletion analysis of B-Myb shows that a specific domain in the ce nter of B-Myb, but not the DNA-binding domain is required for HSE-depe ndent transactivation. We also show that deletion of the C-terminal do main of B-Myb does not affect HSE-dependent transactivation but allows the protein to activate a promoter containing Myb binding sites. This suggests that the ability to activate Myb binding site containing pro moters is repressed in the context of full length B-Myb and that HSE d ependent and Myb binding site dependent transactivation are distinct f unctions of B-Myb. Finally, we report that cyclin D1 like B-Myb strong ly activates the HSP70 promoter via the HSE. HSE-dependent transactiva tion is a novel activity of cyclin D1 and appears to be independent of the phosphorylation of the Rb protein. Our results reveal an interest ing and unexpected connection between HSE-dependent gene activation an d proteins expressed during the G1/S-transition of the cell cycle.