P22 phage

The P22 virus is a lysogenic bacteriophage that infects Salmonella. Its linear, double-stranded DNA genome is 44 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.^[1] 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.

When it is ready to leave the cell, it is packaged into its capsid by the "headful" mechanism, meaning that DNA is packaged into the capsid until it is full. This usually encompasses 48Kb so part of the host DNA is transferred along with the phage. The cell is then lysed, releasing the phage into the environment to infect other cells.

References

1. ^ Snyder L, Champness W (2007). Molecular Genetics of Bacteria (3rd ed.). ASM Press. ISBN 1555813992. 

http://www.sci.sdsu.edu/~smaloy/Research/pdf%20files/P22.pdf