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Virology Lectures 2025: Transcription and RNA Processing

• 0:08 Introduction to Transcription: The lecture focuses on making RNA from DNA templates (transcription) and subsequent RNA processing, specifically for DNA viruses.

• 0:46 DNA Template Specificity: Transcription always occurs from double-stranded DNA templates. Single-stranded or gapped DNA viral genomes must be converted to fully double-stranded DNA before transcription can begin.

• 1:45 Protein Synthesis Precedes Replication: One or more viral proteins are required before DNA viral genomes can be replicated.

• 3:30 Cellular DNA Repair: When broken DNAs enter a cell nucleus, the cell will fix the DNA.

• 5:46 DNA-Dependent RNA Polymerases: Most DNA viruses utilize cellular DNA-dependent RNA polymerases (Pol I, Pol II, Pol III) for transcription. Pol II is crucial for mRNA synthesis. Some viruses, like poxviruses, encode their own because they replicate in the cytoplasm, outside the nucleus.

• 7:52 Cytoplasmic Replication: Viruses that replicate in the cytoplasm need to encode their own RNA polymerase because cellular ones exist only in the nucleus.

• 8:05 Transcription Process Overview: The process includes initiation at promoters (marked by red arrows), 5' capping, creation of a pre-mRNA with exons and introns, splicing (removal of introns), polyadenylation, and export to the cytoplasm for translation.

• 9:46 Transcriptional Regulation: Transcription is highly regulated via sequences in the DNA that bind regulatory proteins.

• 12:23 Regulatory Proteins: The DNA sequences bind regulatory proteins, which participate in regulating transcription.

• 16:28 Enhancers: Enhancers, located far from the transcription start site, enhance initiation by having proteins bind both to it and the initiation complex.

• 17:10 Non-DNA Binding Regulatory Proteins: Some regulatory proteins don't bind DNA directly but modulate histone structure (e.g., through methylation or acetylation) to influence DNA wrapping and transcription.

• 22:48 Transcriptional Regulation Strategies:

  • Cascade Regulation: One gene's product (protein X) is required for the transcription of another gene (gene Y).
  • Positive Auto-regulatory Loops: A gene's product (protein X) enhances the transcription of its own gene.

• 25:16 SV40 Example: A simple example of temporal regulation. Early promoter produces T antigen, which triggers DNA replication and activates the late promoter (for structural proteins).

• 27:26 Late Promoter Activation: The cellular protein IBP inhibits the SV40 late promoter. As viral genomes replicate, IBP becomes limiting, allowing late gene transcription.

• 30:56 Adenovirus Example: More complex regulation. The immediate-early protein E1A is required for transcription of early genes, which are necessary for DNA synthesis.

• 37:31 Herpesvirus Example: The immediate-early promoter is not recognized by the host cell. The virion protein VP16, carried into the cell, allows recognition and initiation of immediate-early transcription.

• 41:43 mRNA Modifications:

  • 5' Cap: A GTP linked via a 5'-5' triphosphate linkage, with methylations, essential for mRNA stability and translation.
  • Poly(A) Tail: Added post-transcriptionally to the 3' end, typically after cleavage of the pre-mRNA.

• 45:36 RNA Splicing: Removal of introns (intervening sequences) from pre-mRNA, discovered in adenovirus-infected cells. Carried out by spliceosomes (ribozymes – RNA enzymes).

• 49:58 Splicing Function: Allows for the production of multiple mRNAs/proteins from a single gene (alternative splicing), expanding the coding capacity of viral genomes.

• 53:54 Splicing and Nuclear Export: Splicing marks mRNAs for export from the nucleus. Proteins that bind during splicing are recognized by nuclear export machinery.

• 55:31 Export of Unspliced RNA: Some viral RNAs (e.g., retroviruses) need to be exported unspliced. They utilize mechanisms like the Constitutive Transport Element (CTE) or Rev protein (HIV) to bypass the splicing-dependent export requirement.

• 58:56 RNA Editing: The hepatitis delta virus provides an example of non-templated RNA editing.

• 1:00:31 Other RNAs: Cells encode many other RNAs besides mRNA, classified as short and long. They include microRNAs and circular RNAs, many of which have regulatory functions.

• 1:03:50 Circular RNAs: These can act as sponges to bind microRNAs and RNA-binding proteins.

• 1:04:09 RNA Methylation: Specifically, N6 methylation of adenosines can influence viral genome replication and packaging (e.g., hepatitis C virus).

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