The Old protein of bacteriophage P2 is responsible for interference with the growth of phage lambda and for killing of recBC mutant Escherichia coli. We have purified Old fused to the maltose-binding protein to 95% purity and characterized its enzymatic properties. The Old protein fused to maltose-binding protein has exonuclease activity on double-stranded DNA as well as nuclease activity on single-stranded DNA and RNA. The direction of digestion of double-stranded DNA is from 5' to 3', and digestion initiates at either the 5'-phosphoryl or 5'-hydroxyl terminus. The nuclease is active on nicked circular DNA, degrades DNA in a processive manner, and releases 5'-phosphoryl mononucleotides.
Sensor:Unknown
Activator:Unknown
Effector:Unknown
Effector:Nucleic acid degrading
PFAM:PF13175, PF13304
contributors:
-Marian Dominguez-Mirazo
relevantAbstracts:
-doi:10.1128/jb.177.3.497-501.1995
-doi:10.1128/jb.177.3.497-501.1995
---
# Old_exonuclease
## Description
The OLD proteins are a family of nucleases present in bacteria, archaea, and viruses :ref{doi=10.1093/nar/gkz703}. The OLD protein found in the P2 *E.coli* prophage is the best characterized one. The protein is an exonuclease that digests dsDNA in the 5' to 3' direction :ref{doi=10.1128/jb.177.3.497-501.1995}. It also has nuclease activity against single stranded DNA and RNA :ref{doi=10.1128/jb.177.3.497-501.1995}. It's been shown to protect against phage lambda :ref{doi=10.1128/jb.177.3.497-501.1995}, and when cloned with the P2 Tin accesory gene, it was shown to protect against other *E. coli* phages :ref{doi=10.1016/j.chom.2022.02.018}. The protein also contains an ATPase domain that affects nuclease activity :ref{doi=10.1128/jb.177.3.497-501.1995}. Inhibition of the RecBCD complex activates the OLD nuclease :ref{doi=10.1016/j.mib.2023.102325}. OLD proteins are divided into two classes based on amino acid sequence conservation and gene neighborhood :ref{doi=10.1093/nar/gkz703}. The P2 associated protein belongs to class 2 :ref{doi=10.1093/nar/gkz703}.
## Molecular Mechanisms
The old_exonuclease is dsDNA exonuclease that digest in the 5' to 3' direction :ref{doi=10.1128/jb.177.3.497-501.1995}. To our knowledge, other aspects of the molecular mechanisms remain unknown.
## Example of genomic structure
The Old_exonuclease system is composed of one protein: Old_exonuclease.
**Rousset, F. et al. Phages and their satellites encode hotspots of antiviral systems. Cell Host & Microbe 30, 740-753.e5 (2022).**
Bacteria carry diverse genetic systems to defend against viral infection, some of which are found within prophages where they inhibit competing viruses. Phage satellites pose additional pressures on phages by hijacking key viral elements to their own benefit. Here, we show that E. coli P2-like phages and their parasitic P4-like satellites carry hotspots of genetic variation containing reservoirs of anti-phage systems. We validate the activity of diverse systems and describe PARIS, an abortive infection system triggered by a phage-encoded anti-restriction protein. Antiviral hotspots participate in inter-viral competition and shape dynamics between the bacterial host, P2-like phages, and P4-like satellites. Notably, the anti-phage activity of satellites can benefit the helper phage during competition with virulent phages, turning a parasitic relationship into a mutualistic one. Anti-phage hotspots are present across distant species and constitute a substantial source of systems that participate in the competition between mobile genetic elements.
