PD-T4-3 is a single gene system isolated during an experimental screen of fosmids derived from 71 strains of diverse E.coli, and cloned back in a derivative of K12. Fragments containing the PD-T4-3 gene were shown to provide resistance to phage T4. The protein is characterized by a GIY-YIG nuclease domain, and mutations inactivating this domain abolished the resistance phenotype. It was originally found on a µ-like prophage
## Molecular mechanism
the molecular mechanism of this system is unknown
## Example of genomic structure
The PD-T4-3 system is composed of one protein: PD-T4-3.