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The nucleotide sequence of an EcoRI duck hepatitis B virus (DHBV) clone was elucidated by using the Maxam and Gilbert method. This sequence, which is 3,021 nucleotides long, was compared with the two previously analyzed hepatitis B-like viruses (human and woodchuck). From this comparison, it was shown that DHBV is derived from an ancestor common to the two others but has a slightly different genomic organization. There was no intergenic region between genes 5 and 8, which were fused into a single open reading frame in DHBV. Genes for the surface and core proteins were assigned to open reading frames 7 and 5/8. Amino acid comparisons showed some structural relationship between gene 6 product and avian reverse transcriptase, suggesting either evolution from a common ancestor or convergence to some particular structure to fulfill a specific function. This should be correlated with the synthesis of an RNA intermediate during DNA replication. This is also taken as an argument in favor of the hypothesis that gene 6 codes for the DNA polymerase that is found within the virion. DNA sequence comparison also showed that the two mammalian hepatitis B viruses are more homologous to each other than they are to DHBV, indicating that DHBV starts to evolve on its own earlier than the two other viruses, as do birds compared with mammals. From this it is proposed that the viruses evolved in a fashion parallel to the species they infect.

The complete nucleotide sequence of a woodchuck hepatitis virus genome cloned in Escherichia coli was determined by the method of Maxam and Gilbert. This sequence was found to be 3,308 nucleotides long. Potential ATG initiator triplets and nonsense codons were identified and used to locate regions with a substantial coding capacity. A striking similarity was observed between the organization of human hepatitis B virus and woodchuck hepatitis virus. Nucleotide sequences of these open regions in the woodchuck virus were compared with corresponding regions present in hepatitis B virus. This allowed the location of four viral genes on the L strand and indicated the absence of protein coded by the S strand. Evolution rates of the various parts of the genome as well as of the four different proteins coded by hepatitis B virus and woodchuck hepatitis virus were compared. These results indicated that: (i) the core protein has evolved slightly less rapidly than the other proteins; and (ii) when a region of DNA codes for two different proteins, there is less freedom for the DNA to evolve and, moreover, one of the proteins can evolve more rapidly than the other. A hairpin structure, very well conserved in the two genomes, was located in the only region devoid of coding function, suggesting the location of the origin of replication of the viral DNA.