Novel short transcripts of hepatitis B virus X gene derived from intragenic promoter

The hepatitis B virus X gene encodes a transcriptional trans-activator that appears essential for viral replication in the infected liver. It has been shown by others that at least two X protein products, initiating at different in-frame ATG codons within the X open reading frame, are necessary for full trans-activating function. Yet, previous studies have demonstrated only a full-length X gene transcript that extends 5' of the first ATG codon in the X open reading frame. In this communication, we present evidence for the existence of shorter X gene transcripts, whose heterogeneous 5' ends straddle the second ATG codon of the X open reading frame. These transcripts are not degradation products of the full-length X gene transcripts but rather arise from an independent promoter. Furthermore, immunofluorescence and functional analyses reveal that active X proteins are translated from these transcripts. Therefore, there appears to be a previously undescribed promoter embedded within the X open reading frame that can give rise to functionally important transcripts.