Question

1. Most viral pathogens have at least early and late genes. Explain the difference between the two types, and give one example of each for a specific viral pathogen (you don’t have to give the gene number, only a description of the purpose of the gene)

Answer 1

The initial stage in the viral replication cycle is expression of the viral early genes which mainly produce proteins for the replciation of the virus. Transcription of these genes occurs using the host's RNA polymerase II and cellular transcription factors. These proteins bind to the viral DNA in regions called early promoters or enhancers, and promote synthesis of the early pre-mRNAs. These early RNAs are processed (capped, polyadenylated and spliced) in the and then transported to the cytoplasm where they are translated, giving rise to the early proteins. These different mRNAs then encode different proteins, as in the example of mouse polyoma virus.

After viral DNA replication has begun, the late genes are transcribed and translated to give rise to late proteins which mainly code for the structural proteins and capside proteins. In the case of the polyomaviruses there are 3 late proteins, VP1, VP2 and VP3. VP1 is the major structural protein of the capsid, followed by VP2 and VP3 which are less abundant, with approximately 30-60 molecules per virion.

Both late and early viral proteins are synthesized in the cytoplasm, but are often transported back to the nucleus where both viral replication and nucleocapsid assembly occurs.