Nhi
Description
The Nhi (nuclease-helicase immunity) system targets and degrades specific phage DNA replication intermediates (N/A) . Nayeemul Bari et al. showed that Nhi from Staphylococcus epidermidis protects against a diverse panel of staphylococcal phages and Millman et al. showed that a protein Nhi-like (that shares the domain organization with Nhi but not the sequence) from Bacillus cereus protects against some Bacillus phages (N/A, N/A) .
Molecular mechanisms
Nhi contains two domains, a nuclease and a helicase domain that are both needed for the anti-phage activity. The nuclease domain has 3′–5′ exonuclease and plasmid nicking activities while the helicase unwinds dsDNA biderctionally. Nhi Clade 1b (pfam01443) specifically recognizes phage single-stranded DNA binding proteins (SSB) that cover the phage genome to target this DNA for degradation thanks to its helicase and nuclease domains (N/A) . In contrast, Nhi Clade 2 (COG0610) specifically recognizes the phage replisome initiator-organizer (RepO), a protein that binds to prophage iterons, catalyzes the initial dsDNA melting, and recruits the replicative helicase (N/A) . The phage proteins recognized by other Nhi-like clades (e.g. Clade 1a/DUF2075, Clade 1c/pfam13538) are unknown. This heterogeneity in phage trigger proteins can result in confusion regarding which phage families the Nhi system can target.
Example of genomic structure
The Nhi is composed of 1 protein: Nhi.
Here is an example found in the RefSeq database:
The Nhi system in Staphylococcus sp. MZ9 (GCF_018622975.1, NZ_CP076029) is composed of 1 protein: Nhi (WP_045177897.1)