cis-Acting sequences 5E, M, and 3E interact to contribute to primer translocation and circularization during reverse transcription of avian hepadnavirus DNA.

Hepadnaviral reverse transcription requires template switches for the genesis of relaxed circular (RC) DNA, the major genomic form in virions. Two template switches, primer translocation and circularization, are required during the synthesis of the second, or plus, strand of DNA. Studies of duck hepatitis B virus (DHBV) indicate that in addition to the requirement for repeated sequences at the donor and acceptor sites, template switching requires at least three other cis-acting sequences, 5E, M, and 3E. In this study we analyzed a series of variant heron hepatitis B viruses (HHBV) in which the regions of the genome that would be expected to contain 5E, M, and 3E were replaced with DHBV sequence. We found that all single and double chimeras were partially defective in the synthesis of RC DNA. In contrast, the triple chimera was able to synthesize RC DNA at a level comparable to that of unchanged HHBV. These results indicate that the three cis-acting sequences, 5E, M, and 3E, need to be compatible to contribute to RC DNA synthesis, suggesting that these sequences interact during plus-strand synthesis. Second, we found that the defect in RC DNA synthesis for several of the single and double chimeric viruses resulted from a partial defect in primer translocation/utilization and a partial defect in circularization. These findings indicate that the processes of primer translocation and circularization share a mechanism during which 5E, M, and 3E interact.