T7 phage

Bacteriophage T7 (or the T7 phage) is a bacteriophage, a virus that infects bacteria. It infects most strains of Escherichia coli and relies on these hosts to propagate. Bacteriophage T7 has a lytic life cycle, meaning that it destroys the cell it infects. It also possesses several properties that make it an ideal phage for experimentation: its purification and concentration have produced consistent values in chemical analyses; it can be rendered noninfectious by exposure to UV light; and it can be used in phage display to clone RNA binding proteins. In a 1945 study by Demerec and Fano, T7 was used to describe one of the seven phage types (T1 to T7) that grow lytically on Escherichia coli; although all seven phages were numbered arbitrarily, phages with odd numbers, or T-odd phages, were later discovered to share morphological and biochemical features that distinguish them from T-even phages. Before being physically referred to as T7, the phage was used in prior experiments. German-American biophysicist Max Delbrück worked with the same virus in the late 1930s, calling it phage δ, and French-Canadian microbiologist Félix d'Herelle likely studied its close relative in the 1920s. T7 grows on rough strains of Escherichia coli (i.e. those without full-length O-antigen polysaccharide on their surface) and some other enteric bacteria, but close relatives also infect smooth and even capsulated strains. E. coli is more resistant to T7 than to some other similar phages. The virus has complex structural symmetry, with a capsid of the phage that is icosahedral (twenty faces) with an inner diameter of 55 nm and a tail 19 nm in diameter and 28.5 nm long attached to the capsid. The ejection of proteins from the capsid upon infection causes the virus to change structure when it enters the cell. The genome of phage T7 was among the first completely sequenced genomes and was published in 1983. The head of the phage particle contains the roughly 40 kbp dsDNA genome which encodes 55 proteins.