Anti-restriction protein Arn (Anti-rgl nuclease; gpSoc.4) is a 92-amino-acid double-stranded DNA-mimic protein of bacteriophage T4 that functions as a competitive inhibitor of host modification-dependent restriction endonucleases, including Escherichia coli McrA (RglA) and McrBC (RglB). Forming a homodimer with an array of negatively charged surface residues that structurally mimic the phosphate backbone of B-form DNA (PDB 3WX4), Arn sequesters host restriction enzymes and prevents cleavage of T4 DNA containing 5-hydroxymethylcytosine. Despite its legacy name "Anti-restriction endonuclease" (meaning inhibitor of restriction endonucleases), Arn possesses no intrinsic nuclease or hydrolase activity.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0004518 nuclease activity | IEA GO_REF:0000043 | REMOVE | Summary: False-positive keyword mapping caused by misinterpreting the legacy name "Anti-restriction endonuclease" as a nuclease rather than an inhibitor of restriction endonucleases. Reason: Arn is a non-catalytic 92-amino-acid DNA-mimic protein (Pfam PF22134; PDB 3WX4) that binds and inhibits host restriction endonucleases (McrA/McrBC); it has no nuclease activity. Supporting Evidence: file:BPT4/arn/arn-deep-research-falcon.md rather than viewing Arn primarily as a catalytic nuclease, Arn is discussed as part of a set of **protein-based inhibitors/mimics** that subvert restriction systems. file:BPT4/arn/arn-uniprot.txt Pays a role in the inhibition of the host restriction-modification system. Strongly inhibits the host mcrA endonuclease that cleaves 5-methyl and 5-hydroxymethylcytosines at the specific DNA sequence C(me)CGG. {ECO:0000269|PubMed:380146, ECO:0000269|PubMed:768783}. |
| GO:0004519 endonuclease activity | IEA GO_REF:0000043 | REMOVE | Summary: False-positive keyword mapping caused by misinterpreting "Anti-restriction endonuclease" as an endonuclease rather than an anti-[restriction endonuclease] inhibitor. Reason: Arn is a non-catalytic DNA-mimic inhibitor of host restriction endonucleases (McrA and McrBC) and does not catalyze phosphodiester bond cleavage. Supporting Evidence: file:BPT4/arn/arn-deep-research-falcon.md rather than viewing Arn primarily as a catalytic nuclease, Arn is discussed as part of a set of **protein-based inhibitors/mimics** that subvert restriction systems. file:BPT4/arn/arn-uniprot.txt Pays a role in the inhibition of the host restriction-modification system. Strongly inhibits the host mcrA endonuclease that cleaves 5-methyl and 5-hydroxymethylcytosines at the specific DNA sequence C(me)CGG. {ECO:0000269|PubMed:380146, ECO:0000269|PubMed:768783}. |
| GO:0016787 hydrolase activity | IEA GO_REF:0000043 | REMOVE | Summary: False-positive keyword mapping derived from the erroneous endonuclease keyword on UniProt P39510. Reason: Arn is a non-enzymatic DNA-mimic protein (Pfam PF22134; PDB 3WX4) that acts by protein-protein interaction to inhibit host restriction endonucleases; it has no hydrolase activity. Supporting Evidence: file:BPT4/arn/arn-deep-research-falcon.md rather than viewing Arn primarily as a catalytic nuclease, Arn is discussed as part of a set of **protein-based inhibitors/mimics** that subvert restriction systems. file:BPT4/arn/arn-uniprot.txt Pays a role in the inhibition of the host restriction-modification system. Strongly inhibits the host mcrA endonuclease that cleaves 5-methyl and 5-hydroxymethylcytosines at the specific DNA sequence C(me)CGG. {ECO:0000269|PubMed:380146, ECO:0000269|PubMed:768783}. |
| GO:0052031 symbiont-mediated perturbation of host defense response | IEA GO_REF:0000043 | MODIFY | Summary: Broad parent term that should be refined to the specific descendant GO:0099018 (symbiont-mediated evasion of host restriction-modification system). Reason: Arn specifically counteracts the host modification-dependent restriction system, which is captured by the descendant term GO:0099018 already annotated to the gene. Proposed replacements: symbiont-mediated evasion of host restriction-modification system Supporting Evidence: file:BPT4/arn/arn-deep-research-falcon.md T7 Ocr is described as structurally mimicking one helical turn of B-form DNA with a negatively charged surface resembling the DNA phosphate backbone, enabling inhibition of restriction and even BREX. The same review explicitly states that “**Similar to Ocr, the coliphage T4 protein Arn … uses the same principle**,” placing Arn in the modern DNA-mimic anti-restriction conceptual framework. file:BPT4/arn/arn-uniprot.txt Pays a role in the inhibition of the host restriction-modification system. Strongly inhibits the host mcrA endonuclease that cleaves 5-methyl and 5-hydroxymethylcytosines at the specific DNA sequence C(me)CGG. {ECO:0000269|PubMed:380146, ECO:0000269|PubMed:768783}. |
| GO:0052170 symbiont-mediated suppression of host innate immune response | IEA GO_REF:0000043 | MODIFY | Summary: General keyword-mapped term that should be refined to GO:0099018 (symbiont-mediated evasion of host restriction-modification system). Reason: The specific bacterial innate defense inhibited by Arn is the host restriction-modification system, captured by the descendant term GO:0099018 already annotated to this gene. Proposed replacements: symbiont-mediated evasion of host restriction-modification system Supporting Evidence: file:BPT4/arn/arn-deep-research-falcon.md T7 Ocr is described as structurally mimicking one helical turn of B-form DNA with a negatively charged surface resembling the DNA phosphate backbone, enabling inhibition of restriction and even BREX. The same review explicitly states that “**Similar to Ocr, the coliphage T4 protein Arn … uses the same principle**,” placing Arn in the modern DNA-mimic anti-restriction conceptual framework. file:BPT4/arn/arn-uniprot.txt Pays a role in the inhibition of the host restriction-modification system. Strongly inhibits the host mcrA endonuclease that cleaves 5-methyl and 5-hydroxymethylcytosines at the specific DNA sequence C(me)CGG. {ECO:0000269|PubMed:380146, ECO:0000269|PubMed:768783}. |
| GO:0099018 symbiont-mediated evasion of host restriction-modification system | IEA GO_REF:0000043 | ACCEPT | Summary: Accurately captures the primary biological function of Arn in inhibiting host modification-dependent restriction endonucleases. Reason: Arn directly binds and inhibits host modification-dependent restriction nucleases (McrA/RglA and McrBC/RglB) by mimicking B-form DNA, protecting 5hmC-containing phage DNA from restriction cleavage. Supporting Evidence: file:BPT4/arn/arn-deep-research-falcon.md T7 Ocr is described as structurally mimicking one helical turn of B-form DNA with a negatively charged surface resembling the DNA phosphate backbone, enabling inhibition of restriction and even BREX. The same review explicitly states that “**Similar to Ocr, the coliphage T4 protein Arn … uses the same principle**,” placing Arn in the modern DNA-mimic anti-restriction conceptual framework. file:BPT4/arn/arn-uniprot.txt Pays a role in the inhibition of the host restriction-modification system. Strongly inhibits the host mcrA endonuclease that cleaves 5-methyl and 5-hydroxymethylcytosines at the specific DNA sequence C(me)CGG. {ECO:0000269|PubMed:380146, ECO:0000269|PubMed:768783}. |
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Download this section (compressed HTML)Q: Beyond McrA and McrBC, does T4 Arn also inhibit other bacterial DNA-binding defense complexes such as BREX, Gabija, or Wadjet in the manner reported for phage T7 Ocr?
Q: What is the high-resolution structure of the Arn-McrA and Arn-McrB complexes?
Experiment: Determine the cryo-EM or crystal structure of T4 Arn bound to Escherichia coli McrA and the McrB DNA-binding domain to compare its binding mode with 5hmC-containing DNA.
Experiment: Screen a panel of cloned bacterial anti-phage defense systems (BREX, Druantia, Wadjet, Zorya) in E. coli with and without ectopic T4 arn expression to define the breadth of Arn's anti-defense activity.
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