FIG. 3.

Determination of the core promoter element for the X gene core promoter 1. (A) Nucleotides within the 21-bp minimal promoter region were individually mutated into three other nucleotides, and the transcription activity of the mutants was assayed in vitro. Primer extension products corresponding to start site 1 are shown with the mutated nucleotides (in lowercase letters) and the nucleotide numbers of the HBV genome. The levels of transcription from mutant templates were determined by densitometry and are shown in each lane as the percentage of transcription activity relative to that of the wild-type promoter. Since the use of HeLa and HepG2 extracts produced somewhat different results for one mutation (1118 C→g), the result for that mutant with HepG2 extract is also shown separately in the bottom right panel. (B) Summary of site-directed mutagenesis. Each construct was assayed at least three times. Promoter activities of mutants are expressed as percentages of the wild-type promoter activity.