Viral Gene Expression and Protein Synthesis

Once a viral genome is inside the cell, the virus must produce messenger RNA and translate it into protein. How it does so depends on the genome type: some viruses are read directly by host ribosomes, others must first be transcribed, and many bring their own enzymes to make and modify their mRNA. Because viruses lack ribosomes, all of them ultimately depend on the host translation machinery.

Definition

Viral gene expression is the regulated production of viral messenger RNA and proteins, encompassing transcription or other routes to mRNA, mRNA modification, and translation of viral proteins on host ribosomes.

Scope

This topic covers the production of viral mRNA (transcription, where required), the modifications such as capping that allow mRNA to be recognised by the cell, the temporal programme of early and late gene expression, and the translation of viral proteins on host ribosomes, including strategies that let viral mRNA compete with cellular messages. It is reference and educational material, not clinical guidance.

Core questions

• How does a virus make messenger RNA that host ribosomes will recognise and translate?
• How is viral gene expression ordered in time into early and late phases?
• How do viruses redirect the host translation machinery toward their own messages?

Key concepts

• Genome type as a determinant of the route to mRNA
• Virus-encoded RNA polymerases
• mRNA capping and recognition by the translation machinery
• Temporal programme: early (regulatory) and late (structural) gene expression
• Cap-dependent translation and internal ribosome entry sites (IRES)
• Host shut-off and competition for ribosomes
• Polyprotein processing by viral proteases

Mechanisms

The route to messenger RNA is set by the genome. Positive-strand RNA can be translated directly; negative-strand RNA and double-stranded RNA viruses carry their own polymerase to transcribe mRNA; DNA viruses are generally transcribed by host or viral polymerases; and retroviruses integrate a DNA copy that is then transcribed. To be translated efficiently, viral mRNAs usually need a 5' cap and other features recognised by the cell, and many viruses encode or borrow the enzymes that add a cap, sometimes by unconventional chemistry. Expression is typically ordered in time, with early genes encoding regulatory and replication proteins and late genes encoding structural components. Translation occurs on host ribosomes through the same elongation, termination, and recycling steps used for cellular messages, and viruses deploy strategies - internal ribosome entry sites, suppression of host translation, or polyprotein processing - to ensure their proteins are made.

Clinical relevance

Virus-specific enzymes of gene expression, such as polymerases and capping enzymes, are distinct from host equivalents and are therefore targets for antiviral drugs. This entry describes the underlying molecular biology at a conceptual level for reference and education; it is not a basis for prescribing, drug selection, or individual patient care.

History

The recognition in the 1970s that different viruses reach the common requirement of making mRNA by different routes underpinned the Baltimore classification and framed the study of viral gene expression. Subsequent molecular work characterised virus-encoded polymerases and capping enzymes, defined the early-late temporal programme of many viruses, and uncovered cap-independent translation through internal ribosome entry, revealing how viruses commandeer the host's protein-synthesis machinery.

Key figures

• David Baltimore
• Bruno Canard
• Thomas Dever
• Rachel Green

Seminal works

• decroly-2012
• dever-green-2012

Frequently asked questions

Why do some viruses carry their own polymerase into the cell?

Host cells cannot transcribe certain viral genomes - for example negative-strand or double-stranded RNA - so those viruses package a polymerase in the virion to make the first messenger RNAs once they enter the cell.

If viruses lack ribosomes, how are their proteins made?

Every virus depends on the host cell's ribosomes to translate its messenger RNA; viruses provide the mRNA and often strategies to favour it, but the protein synthesis itself is carried out by the cell.