A combination of linker scanning mutagenesis and deletional analyses has been used to determine the role of individual DNA:protein binding sites on expression from the hepatitis B virus (HBV) enhancer I-X promoter (map position (mp) 1042-1354, HBV adw2). Linker scanning mutation of the EF-C site caused a 67.5% drop in X promoter activity in HuH7 cells, but had no effect in HepG2 or HepSK cells. Mutation of the E element resulted in an approximately 50% reduction in X promoter activity in HUH7. HepG2, and HepSK cells. Deletional analysis showed that sequences upstream of the EF-C site (mp 1163) were required for full X promoter activity and implicated the NF-1 a site as being sufficient for basal X promoter activity. However, PCR-directed linker scanning mutation of the NF-1 a site did not cause a reduction in X promoter activity, indicating that this site was not an essential component of the X promoter. Taken together, these results indicated that multiple, partially redundant protein:DNA interactions in the enhancer I are essential for full X promoter activity. The lack of an essential basal promoter element supports the suggestion that the two separate HBV enhancer elements (enhI and enhII) were created by integration of the X gene into a primordial enhancer element.
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