The activity of the murine Bax promoter is regulated by Sp1/3 and E-box binding proteins but not by p53.

In human cells the expression of the pro-apoptotic protein Bax appears to be regulated through p53-dependent transcriptional activation. However, in the mouse, p53-deficiency does not affect Bax expression. To shed more light on the transcriptional regulation of the bax gene we have analyzed the murine bax promoter. We find several E-box and Sp1/Sp3 binding sites as well as three putative p53 binding sites that are conserved in the human promoter sequence. We can show that both the Sp1 and the E-box binding sites are necessary for proper regulation of bax transcription and show by genomic DMS footprinting that all these sites are occupied in vivo. In contrast, the putative p53 binding sites were not occupied by protein in vivo in primary murine thymocytes either before or after induction of p53 by DNA damage. Moreover, p53 was unable to regulate the transcription of bax promoter fragments up to 6.5 kb in length. Further, steady state levels of bax mRNA did not correlate with Bax protein expression levels in DNA damage-induced cell death. Our findings exclude a direct transcriptional transactivation of the bax gene by p53 in murine cells suggesting a dominance of p53 independent mechanisms for the regulation of Bax protein expression.