The classification of viral
proteins as early proteins or
late proteins depends on their relationship with
genome
In the fields of molecular biology and genetics, a genome is all the genetic information of an organism. It consists of nucleotide sequences of DNA (or RNA in RNA viruses). The nuclear genome includes protein-coding genes and non-coding g ...
replication. While many viruses (such as
HIV
The human immunodeficiency viruses (HIV) are two species of ''Lentivirus'' (a subgroup of retrovirus) that infect humans. Over time, they cause acquired immunodeficiency syndrome (AIDS), a condition in which progressive failure of the immune ...
) are described as expressing early and
late
Late may refer to:
* LATE, an acronym which could stand for:
** Limbic-predominant age-related TDP-43 encephalopathy, a proposed form of dementia
** Local-authority trading enterprise, a New Zealand business law
** Local average treatment effect, ...
proteins, this definition of these terms is commonly reserved for class I
DNA virus
A DNA virus is a virus that has a genome made of deoxyribonucleic acid (DNA) that is replicated by a DNA polymerase. They can be divided between those that have two strands of DNA in their genome, called double-stranded DNA (dsDNA) viruses, and ...
es. (HIV has two stages of
protein expression but these are not as a result of two stages of transcription surrounding replication but by the production of the
Rev protein which is required for the export of the transcripts of the second set of proteins transcribed form the
cell nucleus.)
Early proteins are those produced following
entry
Entry may refer to:
*Entry, West Virginia, an unincorporated community in the United States
*Entry (cards), a term used in trick-taking card-games
*Entry (economics), a term in connection with markets
*Entry (film), ''Entry'' (film), a 2013 Indian ...
into the host cell but prior to
replication.
The expression of early 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 the
herpesvirus
''Herpesviridae'' is a large family of DNA viruses that cause infections and certain diseases in animals, including humans. The members of this family are also known as herpesviruses. The family name is derived from the Greek word ''ἕρπειν ...
es, these expression periods overlap.
Example
An example of early gene expression is the expression of the small, middle and
large T antigen encoded by the
polyomavirus
''Polyomaviridae'' is a family of viruses whose natural hosts are primarily mammals and birds. As of 2020, there are six recognized genera and 117 species, five of which are unassigned to a genus. 14 species are known to infect humans, while othe ...
. 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 recruits
DNA polymerase
A DNA polymerase is a member of a family of enzymes that catalyze the synthesis of DNA molecules from nucleoside triphosphates, the molecular precursors of DNA. These enzymes are essential for DNA replication and usually work in groups to create ...
and s/s
DNA-binding protein
DNA-binding proteins are proteins that have DNA-binding domains and thus have a specific or general affinity for single- or double-stranded DNA. Sequence-specific DNA-binding proteins generally interact with the major groove of B-DNA, becaus ...
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.
References
{{Viral proteins
Viral protein class