Editing on the genomic RNA of human hepatitis delta virus.

It has been shown previously that during replication of the genome of human hepatitis delta virus (HDV), a specific nucleotide change occurs to eliminate the termination codon for the small delta antigen (G. Luo, M. Chao, S.-Y. Hsieh, C. Sureau, K. Nishikura, and J. Taylor, J. Virol. 64:1021-1027, 1990). This change creates an extension in the length of the open reading frame for the delta antigen from 195 to 214 amino acids. These two proteins, the small and large delta antigens, have important and distinct roles in the life cycle of HDV. To further investigate the mechanism of this specific nucleotide alteration, we developed a sensitive assay involving the polymerase chain reaction to monitor changes on HDV RNA sequences as they occurred in transfected cells. We found that the substrate for the sequence change was the viral genomic RNA rather than the antigenomic RNA. This sequence change occurred independently of genome replication or the presence of the delta antigen. Less than full-length genomic RNA could act as a substrate, but only if it also contained a corresponding RNA sequences from the other side of the rodlike structure, which is characteristic of HDV. We were also able to reproduce the HDV base change in vitro, by addition of purified viral RNA to nuclear extracts of cells from a variety of species.