Phage T7

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Typical podovirus structure. From ViralZone

Bacteriophage T7 is a virulent phage with an icosahedral head, a short noncontractile tail, and a dsDNA genome of 39937 bp.

Phage structure

Similar in structure to T3, each virion of T7 has an icosahedral head (capsid) with a diameter of 60-61 nm and a 2 nm thick shell (the outer layer being made from two forms of the gene 10 protein). The connector has a wider domain at the head and narrows at the tail, but still allows a channel to run through the center (this is closed in mature virions).

Inside the head is the core, which has 8-fold symmetry and is made of 3 proteins (products of genes 14, 15, and 16) and stacked rings.

The tail is short, only 23 nm long, is made of only two proteins (gp11 and gp12), and has 6 tail-fibers, made of gp17 timers, positioned symmetrically near the head of the tail (which also narrows from 21nm to 9nm from head to end).

Genome structure

39937 bases in a single, linear molecule of dsDNA

Gene 1: specifies RNA polymerase; Genes 2-6: affect kinetics of DNA synthesis; Genes 7-17: specify proteins found within partially and fully assembled particles; Genes 18-19: involved in the maturation of the DNA

General specifications of what the genes coded for were found using Amber Mutations.

Life Cycle

Host recognition

Lipopolysaccharides on the external cell wall of the host cell provide recognition for the Phage which orients itself, perpendicular to the cell, using it's 6 tail fibers.

DNA entry

A protrusion of the Phage (the tail) lengthens and attaches the Phage to the Host cell, and proceeds to send the virus' dsDNA through the channel. After the first 850 to 1000 base pairs, the insertion is paused until an injection promoter allows it to continue.

Early gene expression

DNA replication

T7 breaks down the host cell's DNA to its nucleotides for it's own use (using an endonuclease and exonuclease coded by genes 3 and 6, respectively).

Late transcription



See also

Other sources:


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[3] [4]

  1. Studier, FW (1972) Bacteriophage T7. Science 176 367-76 PubMed
  2. Krüger, DH & Schroeder, C (1981) Bacteriophage T3 and bacteriophage T7 virus-host cell interactions. Microbiol. Rev. 45 9-51 PubMed
  3. Serwer, P et al. (2008) Evidence for bacteriophage T7 tail extension during DNA injection. BMC Res Notes 1 36 PubMed
  4. Kemp, P et al. (2005) Changes in bacteriophage T7 virion structure at the initiation of infection. Virology 340 307-17 PubMed