Complementation analysis of mutants defective in different steps of HBV reverse transcription.

The HBV P gene encodes a multifunctional polyprotein which contains the major enzymatic activities required for hepadnaviral reverse transcription (protein primer, DNA polymerase, and RNase H). Mutant HBV genomes unable to synthesize fully functional P gene products were analysed for their potential to be rescued by a second mutant HBV genome that either contained a wild type P gene (intergenic complementation) or a mutation in a functionally different P gene domain (intragenic complementation). This analysis was carried out by cotransfecting two mutants at a time into HepG2 cells and assaying for the production of core particles containing DNA polymerase activity. The results obtained demonstrate the existence of intergenic, but not of intragenic complementation. This indicates that the primary P gene product is not processed before core assembly, and furthermore that there is a rather free mixing of all HBV gene products in the HBV infected cell which can lead to the efficient propagation of defective viral genomes.