After reading this section, you should be able to:
■ Describe in general terms the strategy used by reverse transcribing DNA viruses to synthesize their nucleic acids and proteins
■ Compare the role of reverse transcriptase in the life cycle of a retrovirus to that in the life cycle of a hepadnavirus
Reverse transcribing DNA viruses use reverse transcriptase to replicate their genomes. Members of four virus families fall into this category: Caulimoviridae (e.g., cauliflower mosaic virus and other plant viruses), Metaviridae (fungal, plant, and animal viruses), Pseudoviridae (fungal, plant, protist, and animal viruses), and Hepadnaviridae (e.g., hepatitis B virus and related animal viruses).
Hepatitis B virus (HBV) is one of the best-studied reverse transcribing DNA viruses and is our focus here. HBV virions are 42-nm spherical particles that contain the viral genome. The HBV genome is a circular dsDNA molecule that consists of one complete but nicked strand and a complementary strand that has a large gap; that is, it is incomplete (figure).
The genome is 3.2 kb in length and consists of four partially overlapping, open reading frames that encode viral proteins. Production of new virions takes place predominantly in liver cells (hepatocytes).
After infecting a host cell, the viral-gapped DNA is released into the nucleus. There host repair enzymes fill the gap and seal the nick, yielding a covalently closed, circular DNA. Transcription of viral genes occurs in the nucleus using host RNA polymerase and yields several mRNAs, including a large 3.4-kb RNA known as the pregenome (a plus-strand RNA).
The RNAs move to the cytoplasm, and the mRNAs are translated to produce viral proteins, including core protein and reverse transcriptase. Reverse transcriptase then associates with the plus-strand RNA pregenome and core protein to form an immature core particle. Reverse transcriptase subsequently reverse transcribes the RNA using a protein primer to form a minus-strand DNA from the pregenome RNA.
The genomes of hepadnaviruses are unusual in several respects. The negative strand of the dsDNA molecule is complete but nicked. The enzyme reverse transcriptase is attached to its 59 end. The positive strand is gapped; that is, it is incomplete (shown in purple). A short stretch of RNA is attached to its 59 end (shown in green). Upon entry into the host nucleus, the nick in the negative strand is sealed and the gap in the positive strand is filled, yielding a covalently closed, circular dsDNA molecule
After almost all the pregenome RNA has been degraded by the RNAseH activity of reverse transcriptase, the remaining RNA fragment serves as a primer for synthesis of the gapped dsDNA genome, using the minus-strand DNA as template. Finally, the nucleocapsid is completed and progeny virions are released
Reverse Transcribing DNA Viruses
■ There are four groups of DNA viruses that use reverse transcriptase in their life cycles. One group, hepadnaviruses, includes hepatitis B virus, which has a dsDNA genome that consist of one complete but nicked strand and an incomplete (i.e., gapped) complementary strand (figure).
■ Upon infection with hepatitis B virus, the host cell repairs the gap and seals the nick to generate a covalently closed, circular viral genome.
■ This serves as the template for the synthesis of pregenome RNA, which is the template for reverse transcription. Reverse transcription produces the dsDNA, gapped genome.