The classification of viralproteins asearly proteins orlate proteins depends on their relationship withgenomereplication. While many viruses (such asHIV)[1] are described as expressing early andlate proteins, this definition of these terms is commonly reserved for class IDNA viruses. (HIV has two stages ofprotein expression but these are not as a result of two stages of transcription surrounding replication but by the production of theRev protein which is required for the export of the transcripts of the second set of proteins transcribed form thecell nucleus.)
Early proteins are those produced followingentry into the host cell but prior toreplication.[1] The expression ofearly genes, commonly encoding non-structural proteins, initiates replication of the genome and expression of late genes.
In some, simpler viruses, this pattern of expression is clearly defined, while in those with more complex genomes, such as theherpesviruses, these expression periods overlap.
An example of early gene expression is the expression of the small, middle andlarge T antigen encoded by thepolyomavirus. The middle T antigen is not required for replication and it acts to enhance transcription by binding host proteins which interact with the late promoter. On the other hand, the large T antigen is required and it acts to initiate replication directly. It binds the viral origin of replication and recruitsDNA polymerase and s/sDNA-binding protein such that once its concentration is great enough it blocks the transcription of early genes and initiates genome replication. It also acts to cause the entry of the host cell into S phase.
Bacteriophage T4 is a virus that infects the bacteriumE. coli. Bacteriophage T4genes are conventionally classified as early function genes or late function genes based on the time period in which their protein products are expressed during the course of bacteriophage infection. In general, the early proteins produced by early function genes act catalytically to promote bacteriophagegenome specificDNA replication andrepair as well as to facilitate modification of the hostnucleotide pool to accommodate these functions.[2] The late function genes code for late proteins that are involved inmorphogenesis of the bacteriophage virion, particularly non-enzymatic proteins that comprise the structural components of the virus itself, but also some catalytic proteins that facilitate this morphogenetic assembly process.[2]