P22 phage

The P22 virus is a bacteriophage, or a virus that infects bacteria - in this case Salmonella. Its circular, double-stranded DNA genome is about 40 kilobases long. Like many phage viruses, it has been used in molecular biology to induce mutations in cultured bacteria and to introduce foreign genetic material. The viral capsid has been the subject of studies in virus assembly. Its "tailspike" protein, which is anchored in the viral coat and used to aid in penetrating the membranes of host cells, has an unusual beta helix fold.