Module 8 - Reverse Transcriptase DNA Viruses

Reverse-transcribing dsDNA viruses use

an RNA intermediate and reverse transcriptase during genome replication

Non-reverse-transcribing dsDNA viruses replicate their genomes…

directly from DNA without an RNA stage

Hepatitis has an unusual genome because

it is typically circular but partially double stranded. Small RNA pieces bound to one end and reverse transcriptase at the other end.

Reverse transcriptase creates new viral genomes from

pre-genomic RNA within the viral core

Hepatitis B genome and icosahedral capsid together from the

nucleocapsid, enclosed within a lipid envelope studded with viral surface proteins

Unlike retroviruses, all hepadnaviral DNA is produced by

reverse transcription

Hepatitis B basic replication scheme

All RNA molecules in hepatitis B replication scheme are produced by

host polymerases

RNA molecules in hepatitis B divide into two groups:

pre-genomic RNA (pgRNA) and mRNA

Pre-genomic RNA in hepatitis B

provides template for new genome production. About 3.4 kb, longest RNA generated by host cellular polymerases

mRNA in hepatitis B

come in various lengths, frameshifts allows for production of multiple overlapping mRNA molecules

pgRNA encodes 

capsid protein (HBcAg) and polymerase

Overlapping transcripts allow hepatitis B to generate multiple

cultural and regulatory proteins from its compact genome

Hepatitis B infects hepatocytes in the

liver

Replication cycle of hepatitis B

1. virus binds to heparan sulfate proteoglycans as attachment factors

2. then binds to sodium taurocholate co-transporting polypeptide (NTCP)

3. after receptor binding, virus enters via receptor-mediated endocytosis and travels to the nucleus

4. at nucleus, viral core binds nuclear pore proteins

5. virus releases the genome into the nucleus

6. host enzymes fill in the gap, forming ds, circular DNA called covalently closed circular DNA (cccDNA)

7. host nuclear polymerases acts on the cccDNA to make pgRNA and mRNAs

8. hepatitis B DNA is copied in the cytoplasm because it has pre-packaged reverse transcriptase (P protein)

9. pgRNA and mRNA is exported from nucleus to cytoplasm

10. in cytoplasm, pgRNA joins with capsid proteins and reverse transcriptase (p protein)

11. RT binds pgRNA and capsid forms around this complex

12. Within the capsid, RT produces a gapped, circular, ds DNA copy of pgRNA

In the replication cycle for hepatitis B, creation of new, partially ds, circular genomes from pgRNA occurs in

the viral core

pgRNA transcribed in the nucleus becomes enclosed within

the forming viral capsid along with polymerase (P protein)

In hepatitis B, the polymerase has multiple domains. Terminal protein (TP) domain initiates _______, while RT domain carries out _________

priming; DNA synthesis