Abstract
The regulatory elements involved in transcription of the C gene of duck hepatitis B virus (DHBV) were investigated. Several DHBV DNA fragments were assayed for C gene promoter, enhancer, and silencer activity by using a chloramphenicol acetyltransferase (CAT) reporter gene and transfection of established liver and nonliver cell lines. A major transcript initiating at nucleotide positions 2532 and 2533 and three minor transcripts initiating at positions 2453/2454 and 2461 were identified in cells containing these constructs. These positions correspond to the 5' end of the C mRNA and were close to that of the pre-C mRNAs, respectively, found in infected livers. The pre-C mRNAs were only detected when sequences located between the initiation sites of the pre-C and C mRNAs were deleted. These sequences downregulated, in an orientation-independent fashion, a heterologous promoter and were found to contain a consensus motif common to negative transcriptional regulatory elements previously characterized in other cellular and viral genes. C gene promoter activity was only observed in highly differentiated liver cells and was dependent on a short DHBV DNA fragment containing an enhancer core consensus motif. These data indicate that transcription of the DHBV C gene is regulated by positive, negative, and differentiation factor-responsive elements.