1. VIRAL GENE EXPRESSION
DR.SOBIA MANZOOR
LECTURE 05

2. Viral Penetartion
Cell penetration is generally an energy dependent process. the cell must be metabolically active for this to occur.
Three main mechanisms are involved:
1) Translocation of the entire virus particle across the plasma membrane.
-rare amongst viruses and poorly understood.
2) Endocytosis of the virus into intracellular vesicles (endosomes).
- very common mechanism, understood in great detail for some viruses
3) Fusion of the virus envelope with a cellular membrane.
- only applicable to enveloped viruses.
- requires the presence of a specific fusion protein in the virus envelope.
Two types of virus-driven membrane fusion:
- pH independent (can occur at cell surface or within an endosome)
- pH dependent (occurs within an acidified endosome)

3. Viral Penetration

4. Uncoating
Uncoating is a general term for the events that follow penetration, virus capsid is completely or partially removed and the virus genome is exposed.

5. Uncoating

6. Viral Fusion

7. Gene Expression DNA viruses:
These viruses must transcribe mRNA using the (-) strand of the DNA genome as a template.
For most DNA viruses the enzyme that performs this function is the host RNA polymerase II. This takes place in the nucleus of infected cells where RNA polymerase II is located.
Notable exceptions are the poxviruses, which replicate in the cytoplasm of infected cells and encode their own RNA polymerase.

8. RNA Viruses
(ii) (+) stranded RNA viruses: The genomes of these viruses can be translated directly by cellular ribosomes.
- Important exception are the retroviruses.
(iii) (-) stranded RNA viruses and double-stranded RNA viruses: mRNA must be transcribed from the genome in order to have gene expression. This function is performed by RNA-dependent RNA polymerases.
Animal cells do not contain this enzymatic function, these viruses must bring this enzyme with them into the infected cell. These viruses package a virus encoded RNA-dependent RNA polymerases into the virus particle.

9. The retroviruses (e. g. HIV), are (+) RNA viruses
The retroviruses (e.g. HIV), are (+) RNA viruses. These viruses replicate through a DNA intermediate:
1) The RNA genome is “reverse transcribed” by the viral enzyme
Reverse Transcriptase into double stranded DNA.
2) This double stranded DNA integrates into the genome of the host cell.
3) The integrated DNA or provirus then serves as a template for the synthesis of viral mRNAs using host RNA polymerase II.
4) This enzyme also synthesizes genomic (+) RNAs using the provirus as a template.

10. Replication of DNA virus

11. Viral Replication (RNA Viruses)

12. Replication Retro Viruses

13. Assembly and Egress
Packaging of the genome into the capsid can occur in one of two ways:
- The capsid assembles around the virus genome or
- The genome is “fed” into preformed capsids
For helical nucleocapsids RNA viruses, viral genomic RNA is generally complexed with nucleocapsid protein during its synthesis.

14. Assembly and Egress Egress
Egress (or exit) from the infected cell is very different for viruses that have an envelope versus those that do not.
(i) Viruses with “naked” capsids often are released from the infected cell by lysis. Poorly understood for most viruses.
Invariably, this results in cell death.
(ii) Viruses that have envelopes can acquire their envelope from a variety of cellular membranes by a process called budding.
Viruses that bud into the plasma membrane are released into the extracellular environment directly.
Viruses that bud into the membranes of the Golgi apparatus or endoplasmic reticulum are secreted from the infected cell.

15. Herpesvirus envelopment and egress