Lecture Notes: Reverse Transcription and Integration

Introduction

• Theme of the Day: Reverse Transcription and Integration
• Quote Introduction: "One can't believe in impossible things..." from Alice in Wonderland reflects the seemingly impossible nature of reverse transcription.

Historical Context

• 1908: Chicken leukemia virus discovered, filtered to induce leukemia in chickens.
• 1911: Rous discovers virus causing sarcoma, leading to Nobel Prize 55 years later.
• Howard Temin's Provirus Hypothesis: RNA tumor viruses transform cells by making DNA copies integrated into the cell genome.
• David Baltimore: Proposed necessity of an enzyme in particles to convert RNA to DNA, leading to discovery of reverse transcriptase.

Discovery of Reverse Transcriptase

• Early 1970s: Temin and Baltimore independently discover RNA-dependent DNA polymerase (reverse transcriptase) in RNA tumor viruses.
• Scientific Etiquette: Baltimore's decision to notify Temin of his upcoming publication.

Impact of Reverse Transcriptase

• Molecular Biology: Revolutionized by enabling techniques like RT-PCR.
• Commercial Availability: Enzyme commercially available under names like "Superscript 3."
• Baltimore Scheme: Retroviruses have positive RNA genomes copied to DNA, integrating into host cells.

Retrovirus Structure and Genome

• Virus Structure: Enveloped virus with glycoproteins, icosahedral capsid, and RNA genome with multiple proteins (gag, pol, env).
• Provirus: Integrated form of the viral genome (DNA) in the host cell.
• Reverse Transcriptase: Functions as RNA-dependent DNA polymerase and RNase H.

Reverse Transcription Process

• Priming: Initiated by cellular tRNA bound to primer binding site (PBS).
• Template Jumping: Reverse transcriptase switches templates to recreate long terminal repeats (LTRs).
• DNA Synthesis: Produces double-stranded DNA (dsDNA) with LTRs.

Integration into Host Genome

• Integration Process: Involves removal of LTR bases and duplication of host DNA at integration sites.
• Non-Random Integration: Preferentially occurs in genes or transcription start sites.

Endogenization and Retroelements

• Endogenous Retroviruses (ERVs): Integrated viral DNA in germ line, passed to offspring, leading to widespread presence in genomes.
• Retrotransposons: Mobile genetic elements moving via reverse transcription.
• Role in Evolution: ERVs contribute functions like the syncytin gene crucial for placenta formation.

Examples and Applications

• Koala Retrovirus: Current example of endogenization in real-time.
• Human Genome: Contains various retroelements, significant genomic component.
• HERV-K: Involved in human development and potential immune system roles.

Hepadnaviruses

• Unique Features: DNA viruses encoding reverse transcriptase with partially double-stranded DNA.
• Replication Cycle: Begins reverse transcription in cytoplasm before viral release.

Summary

• Reverse Transcriptase's Role: Central to retroviral replication and evolutionary processes.
• Genomic Integration: Leads to persistent presence in host genomes with varied biological implications.

Future Topics

• Upcoming Lectures: Focus on viral assembly and more detailed exploration of HIV and related viruses.