Phage T4

The T4 phage or T4 bacteriophage is a type I virus with DNA that infects Escherichia coli bacteria. It has a size of approximately 200 nm. Phage T4 belongs to group T, which also includes enterobacteriophages T2 and T6. Phage T4 has a lytic life cycle only, and not a lysogenic one.

Its DNA is double-stranded and linear, measures 169 kb and can encode up to 289 kinds of proteins.

In order to preserve its DNA by avoiding the action of the host cell's nucleases, it contains different nitrogenous bases than the usual ones, such as cytidine and hydroxymethylcytidine. In this way, the virus can use its own nucleases to destroy the cellular DNA of its host, the bacteriaE. coli.

Its structure is complex. It consists of a capsomere (with DNA inside) or head, attached to a collar and a tail. The tail has at its final end a basal plate, in which the parasites reside (lower part of the plate) and to which the fibers of the tail are attached, which allow the phage to attach to the bacteria. A type of 'whiskers' appear on the collar, environmental sensors that prevent phage infection in unfavorable conditions by regulating the retraction of the tail fibers. All structures are protein.

The T4 phage has some unique characteristics such as:

• High DNA copy speed with only one error every 300 copies.
• Special repair mechanisms.
• Terminal floods that can be recombined and form concathemers. The DNA of the mature viriones is the result of the cut of the concathemers in the units that form it, the genome of the virus.

Replicative cycle

Its life cycle (from entering the bacteria until its destruction) lasts about 30 minutes (at 37 °C) and consists of:

• Adsorption and penetration (it starts immediately).
• Immediate precocious genes are transcribed (it starts immediately).

• Delayed precocious genes are transcribed (it starts after 2 minutes).
• Degradation of the genome E. coli by viral enzymes.
• Transcription of late genes (it starts after 5 minutes).
• Replication of DNA (it starts after 10 minutes).

• End transcription of precocious genes (after 12 minutes).
• Virus formation (the capside is assembled, the tail is joined and finally the tail fibers (it starts after 12 minutes).

Control of transcription

It is carried out by, among others, the following proteins:

• Alt and Mod: they carry out the ADP ribosilation of the Arg265 of the alpha subunit of the polymerase RNA.

• MotA: Joins DNA to activate transcription of delayed precoces genes.

• AsiaA: Blocks transcription of immediate precoces genes and almost all cell genes.

• gp55: alternative σ factor. Positive transcription control.

• gp33: protein bridge between polymerase RNA and gp55. Positive transcription control.

Introns

The T4 phage has introns, which is a very rare characteristic among viruses. They are present in the td, nrdB and sunY genes. They are self-splicing introns and RNA acts as an enzyme catalyzing transesterification reactions and transfer of phosphodiesteric bonds.

The introns present in the td and sunY genes are mobile and can catalyze their transfer to another DNA molecule.

Other T phages

• T1
• T2
• T3
• T5
• T6
• T7