Viral expression strategies

• Absolute dependency on cells for survival and multiplication

• Small genomes – limited coding capacities

~10^3 virus (- 10^6)

• Density of information

• Temporal coordination of the production of viral components

• Successful competition for cellular resources

• Long-term survival of the virus family

Production of viral mRNAs is essential for the production of new viruses (= virus progeny)

production of the mRNA in the nucleus

• HIV provirus

• Long Terminal Repeat

• 5’LTR control region of the provirus contains single promoter/enhancer;

• upstream 5' LTR element was the major transcriptional promoter.

• The RNA “Cap” is essential for the translation of most mRNAs in eukaryotic cells

Leaky scanning

• Leaky scanning is a phenomenon in which a weak initiation codon triplet on the mRNA is sometimes skipped by the ribosome during translation initiation. The ribosomal subunit 40S continues scanning to another initiation codon. The weak initiation codon can be an ACG or an ATG in a weak Kozak consensus context.In this way, one mRNA can code for several different proteins if the AUG are not in frame, or for proteins with different N-termini if the AUG are in the same frame.

• Several viruses use a leaky scanning mechanism to produce vital proteins which implies that leaky scanning is not a consequence of inadequacy, but instead allows viruses to overcome the high selective pressures of competing with their hosts

the former generates a specific protein while the latter generates a number of functionally relevant proteins.

Internal ribosome entry sites (IRESs) are sequences within the RNA molecule that allow binding of ribosomes without scanning

Poliovirus infection blocks attachment of 40S ribosome subunit to the 5‘ end of mRNA => no TK production

Co-expression of selection or marker proteins with proteins of interest

Ribosomal frameshifting: A process by which ribosomes shift into a different reading frame during translation and continue translation in that frame.

Enables production of polyproteins consisting of protein domains translated from different reading frames.

• Stimulatory element: Pseudoknot or

  Stable Stem-Loop Structure —> Ribosome pauses during elongation —> Ribosome shifts its position in 5’ direction

  —> Frameshift; Pseudoknot or stem-loop structure dissolves

Ribosomal Frameshifting

Ribosomal frameshifting —> different Structures of RNA pseudoknots in different Coronaviruses .