The classification of viral

proteins Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, re ...

as early proteins or

late proteins A late protein is a viral protein that is formed after replication of the virus. One example is VP4 from simian virus 40 (SV40). In Human papillomaviruses In Human papillomavirus (HPV), two late proteins are involved in capsid formation: a majo ...

depends on their relationship with

genome 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 genes, other functional regions of the genome such as ...

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 im ...

) are described as expressing early and

late Late or LATE may refer to: Everyday usage * Tardy, or late, not being on time * Late (or the late) may refer to a person who is dead Music * ''Late'' (The 77s album), 2000 * Late (Alvin Batiste album), 1993 * Late!, a pseudonym used by Dave Groh ...

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 t ...

es. (HIV has two stages of

protein expression Protein expression may refer to: *Gene expression, the processes that convert the information of DNA genes into a functional copies of mRNA in living cells *Protein production Protein production is the biotechnological process of generating a ...

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 The cell nucleus (; : nuclei) is a membrane-bound organelle found in eukaryote, eukaryotic cell (biology), cells. Eukaryotic cells usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have #Anucleated_cells, ...

.) 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 ''Orthoherpesviridae'', previously named and more widely known as ''Herpesviridae'', is a large family of DNA viruses that cause infections and certain diseases in animals, including humans. The members of this family are commonly known as herp ...

es, these expression periods overlap.

Examples

An example of early gene expression is the expression of the small, middle and

large T antigen The large tumor antigen (also called the large T-antigen and abbreviated LTag or LT) is a protein encoded in the genomes of polyomaviruses, which are small double-stranded DNA viruses. LTag is gene expression, expressed early in the infectious cycl ...

encoded by the

polyomavirus ''Polyomaviridae'' is a family of DNA viruses whose natural hosts are mammals and birds. As of 2024, there are eight recognized genera. Fourteen species are known to infect humans, while others, such as Simian Virus 40, have been identified i ...

. 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 t ...

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, becau ...

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

Bacteriophage T4 Escherichia virus T4 is a species of bacteriophages that infect ''Escherichia coli'' bacteria. It is a double-stranded DNA virus in the subfamily '' Tevenvirinae'' of the family '' Straboviridae''. T4 is capable of undergoing only a lytic li ...

is a virus that infects the bacterium ''E. coli''. Bacteriophage T4

gene In biology, the word gene has two meanings. The Mendelian gene is a basic unit of heredity. The molecular gene is a sequence of nucleotides in DNA that is transcribed to produce a functional RNA. There are two types of molecular genes: protei ...

s 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 bacteriophage

specific

DNA replication In molecular biology, DNA replication is the biological process of producing two identical replicas of DNA from one original DNA molecule. DNA replication occurs in all life, living organisms, acting as the most essential part of heredity, biolog ...

and

repair The technical meaning of maintenance involves functional checks, servicing, repairing or replacing of necessary devices, equipment, machinery, building infrastructure and supporting utilities in industrial, business, and residential installat ...

as well as to facilitate modification of the host

nucleotide Nucleotides are Organic compound, organic molecules composed of a nitrogenous base, a pentose sugar and a phosphate. They serve as monomeric units of the nucleic acid polymers – deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), both o ...

pool to accommodate these functions. The late function genes code for late proteins that are involved in

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

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.

References

{{Viral proteins Viral protein class