Phenobarbital responsiveness conferred by the 5′-flanking region of the rat CYP2B2 gene in transgenic mice

Phenobarbital (PB) is a prototype for a class of agents that produce marked transcriptional activation of a number of genes, including certain cytochrome P-450s. We used transgenic mouse approaches and multiple gene reporters to assess the functional consequences of specific deletions and site-specific mutations within the 2.5 kb 5′-flanking region of the rat CYP2B2 gene. Protein–DNA interactions at the PBRU domain also were characterized. Using the transgenic models, we demonstrate that sequences between −2500 and −1700 bp of the CYP2B2 gene are critical for PB induction; mice with 1700 or 800 bp of 5′-flanking CYP2B2 sequence are not PB responsive. DNA affinity enrichment techniques and immunoblotting and electromobility shift assays were used to determine that nuclear factor 1 (NF-1) interacts strongly with a site centered at −2200 bp in the PB responsive unit (PBRU) of CYP2B2. To test the functional contribution of NF-1 in PB activation, we introduced specific mutations within the PBRU NF-1 element and demonstrated that these mutations completely ablate the binding interaction. However, transgenic mice incorporating the mutant NF-1 sequence within an otherwise wild-type −2500/CYP2B2 transgene maintained full PB responsiveness. These results indicate that, despite the avidity of the respective DNA–protein interaction within the PBRU in vitro, NF-1 interaction is not an essential factor directing PB transcriptional activation in vivo.