| Verified identity | Molecular function claim | Evidence type | Key experimental observations and quantitative data | Primary citations with URL and year |
|---|---|---|---|---|
| **Arn / urn** from **Enterobacteria phage T4**; anti-restriction endonuclease / anti-Rgl nuclease; T4-relative review notes Arn is the factor that counters host **Mcr (Rgl)** restriction and that **arn alleles occur in all T-even phages and CC31** (pqac-00000004, pqac-00000011) | Best-supported function is **antagonism of host Rgl/Mcr restriction of T4 DNA**, specifically the activity historically called **anti-restriction endonuclease (urn/arn)** that inactivates host restriction acting on nonglucosylated T4 DNA; exact catalytic chemistry/substrate cleavage reaction for purified Arn is **not established in the retrieved evidence** (pqac-00000003, pqac-00000004, pqac-00000011) | Genetic, review, comparative genomics/bioinformatic (pqac-00000003, pqac-00000004, pqac-00000011) | In restricting hosts, **T4 glu-** genomes fail to express arn when parental DNA is cleaved by **rgl**; authors conclude arn expression is **directly inactivated by restriction endonuclease cleavage**. In **rgl+ ExoV+** cells superinfected with **32P-T4 glu-**, DNA breakdown after primary infection with **T4 glu- imm-** reached **58%**, versus **5%** when primary phage was **T4 glu+ imm-**; in **rgl+ ExoV-** hosts, corresponding breakdown was **10%** vs **3%** (pqac-00000001, pqac-00000003, pqac-00000010) | Dharmalingam & Goldberg, *Virology* (1979), https://doi.org/10.1016/0042-6822(79)90098-9 ; Petrov et al., *Virology Journal* (2010), https://doi.org/10.1186/1743-422x-7-292 |
| Same T4 Arn/urn identity; distinct from **arx/urx**, the anti-ExoV activity, which is a separate phage anti-restriction function (pqac-00000000, pqac-00000009) | Arn acts in the **restriction-alleviation pathway** directed against host **r2,4/Rgl-type restriction**, whereas **Arx** inhibits host **ExoV**; this distinction is critical because some older literature discusses both activities together (pqac-00000000, pqac-00000009) | Genetic and biochemical cell-extract assays (pqac-00000000, pqac-00000009) | In **T4 glu+** infection of **rgl+ ExoV+** cells, host **ExoV** activity was inhibited by about **99% within 5 min**; in **T4 glu-** infection, about **80%** of ExoV activity remained. Mixing assays showed **T4 glu+** extracts reduced residual ExoV activity to **6-10%**, whereas **T4 glu-** extracts from **rgl+ ExoV+** cells showed **100%** residual activity. These data support that **arx**, not arn, is the anti-ExoV factor, preventing misannotation of Arn as an ExoV inhibitor (pqac-00000000, pqac-00000009) | Dharmalingam & Goldberg, *Virology* (1979), https://doi.org/10.1016/0042-6822(79)90098-9 |
| T4 Arn/urn is linked functionally to T4 DNA glucosylation state; T4 relatives use glucosylated hydroxymethylcytosine to resist host restriction, with Arn adding an extra anti-restriction layer against **Mcr/Rgl** systems (pqac-00000004, pqac-00000011) | Biological role is **early protection of infecting phage DNA from host restriction**, especially when DNA is vulnerable because it lacks protective glucosylation; Arn therefore belongs to the broader **phage anti-defense / anti-restriction arsenal** (pqac-00000004, pqac-00000011) | Review and comparative genomics (pqac-00000004, pqac-00000011) | Review evidence states glucosylation of T-even DNA makes it resistant particularly to **E. coli Mcr (Rgl)** and lists **arn** among phage genes involved in genome modification/restriction avoidance. Arn distribution is restricted: **all T-even type phages and phage CC31 only** among surveyed T4 relatives (pqac-00000011) | Petrov et al., *Virology Journal* (2010), https://doi.org/10.1186/1743-422x-7-292 |
| UniProt target identity is therefore consistent with the literature concept of **T4 anti-Rgl / anti-restriction endonuclease Arn (urn)**, but literature coverage is sparse and nomenclature is old/ambiguous; some sources mention Arn as a **protein mimic of DNA** in the context of hypermodified DNA review (pqac-00000005) | Current understanding: **function is established genetically**, but **direct biochemical mechanism, structure, and subcellular localization remain poorly resolved in the retrieved literature**; likely acts in the **infected-cell cytoplasmic DNA context early after genome entry**, because its benefit would have to precede irreversible restriction (pqac-00000001, pqac-00000005, pqac-00000010) | Genetic inference, review, bioinformatic/interpretive (pqac-00000001, pqac-00000005, pqac-00000010) | Dharmalingam & Goldberg argue that direct inactivation of arn by restriction cleavage implies anti-restriction would need to act **very early** to protect incoming unmodified phage DNA. The hypermodified-DNA review notes support for T4 Arn as part of anti-restriction strategy, but does not provide mechanistic enzymology; thus claims beyond anti-Rgl function should be treated cautiously (pqac-00000001, pqac-00000005, pqac-00000010) | Dharmalingam & Goldberg, *Virology* (1979), https://doi.org/10.1016/0042-6822(79)90098-9 ; Hutinet et al., *EcoSal Plus* (2021), https://doi.org/10.1128/ecosalplus.esp-0028-2019 |


*Table: This table summarizes the best-supported functional annotation for Enterobacteria phage T4 Arn/urn, separating it from the distinct anti-ExoV factor Arx and highlighting the strongest quantitative evidence available. It is useful for verifying gene identity and for understanding what is experimentally established versus still uncertain.*