Repression of cytochrome P‐450c gene expression by cotransfection with adenovirus E1a DNA

Gene expression of rat cytochrome P‐450c (P‐450c) depends upon inducible enhancers scattered in the 5′‐upstream region of the gene. We show that expression of the P‐450c gene is repressed by contransfection with adenovirus E1a DNA, regardless of the presence or absence of inducers, in a transient expression system of HeLa cells. Since cotransfection of either 13S or 12S E1a cDNA was effective in the repression, the region necessary for repression could be separated from that of transactivation of other adenovirus early genes. Moreover, we investigated the regions responsible for the inhibitory activity using in‐frame deletion mutants lacking internal or external portions of the E1a proteins. The sequence responsible for the repression was located in the aminoterminal half of the E1a proteins. The inducible expression of the chimeric plasmid containing a 24‐base‐pair enhancer sequence of the P‐450c gene placed in a heterologous promoter of SV40 was repressed by contransfection with E1a DNA, suggesting strongly that the inhibitory effect of the E1a proteins upon P‐450c gene expression was caused by blocking the enhancer activity.