Mnt is a repressor which is involved in the genetic switch between lysogenic and lytic growth in bacteriophage P22. The C-terminal domain of the protein consists of a dimer of two antiparallel coiled coils with a right handed twist, which is both stronger and has closer inter-helical separation compared with those found in left-handed coiled coils [].This entry is represented by Bacteriophage P22, Mnt. The characteristics of the protein distribution suggest prophage matches in addition to the phage matches.
This superfamily represents a domain with a ribbon-helix-helix topology consisting of four helices in an open array of two hairpins. This domain is found in Mnt and Arc, two structurally homologous repressors encoded by bacteriophage P22 [, ]. This entry also includes nickel-responsive regulator []and protein CopG [].
The Antirepressor protein ant from Salmonella phage P22 prevents the prophage p22 c2 repressor protein from binding to its operators. It also inhibits the action of other prophage repressor proteins, including those of phages lambda and 434. The synthesis of antirepressor is negatively regulated by the protein products of the two other immi genes, mnt and arc [, , ]. This entry represents the N-terminal domain of this protein and similar proteins from tailed bacteriophages (Caudovirales) and bacterial prophages mostly found in Proteobacteria.
Detailed sequence analysis suggests that members of this family represent the minimal domain of the PolB nucleotidyltransferase superfamily []. The conservation of the nucleotidyltransferase core and particularly the negatively charged metal-chelating residues lead to the prediction that members of this family possess nucleotidyltransferase activity. Their small size, however, leaves very little beyond the core catalytic domain to help in specific substrate recognition as seen in other, larger members of the PolB superfamily []. The conserved region of the MNTs ("minimal"nucleotidyltransferases) includes approximately 90 amino acid residues that correspond to the core domain of kanamycin nucleotidyltransferase []. A structural model of the MNT domain has been proposed [].
This superfamily represents domains with a ribbon-helix-helix core topology consisting of four helices in an open array of two hairpins. Such domains are found in several bacterial and phage repressors, including the Escherichia coli methionine repressor (MetJ), which when combined with S-adenosylmethionine (SAM) represses the expression of the methionine regulon and of enzymes involved in SAM synthesis []. Other bacterial and phage repressors containing domains with a similar fold include the bacterial plasmid-encoded repressors CopG (), the bacterial omega transcription repressor [], and the phage repressors Arc []and Mnt []. These repressors are usually obligate dimers, which pair through a single N-terminal strand, and possess a C-terminal helix-turn-helix unit [].
This entry describes prokaryotic proteins including tRNA nuclease HepT from Aphanizomenon flos-aquae, the toxic component of a type II toxin-antitoxin (TA) system comprising a minimal nucleotidyltransferase (MNT) and the accompanying HEPN (higher eukaryotes and prokaryotes nucleotide-binding) protein []. HepT dimerises and enables the formation of a deep cleft at the HEPN-domain interface, containing the RX4-6H motif (where X is any amino acid and the residue immediately after the conserved R is typically a polar amino acid), the active site that functions as an RNA-cleaving RNase []. HepT from Aphanizomenon flos-aquae targets a subset of tRNAs and cuts off 4 nt from the 3' end of tRNA acceptor stem, but its toxicity is neutralized by covalent di-AMPylation performed by the MNT antitoxin [].
This family includes the toxic component HepT of a type II toxin-antitoxin (TA) system, which has RNase activity. These proteins contain a HEPN (higher eukaryotes and prokaryotes nucleotide-binding) domain and are neutralized through tri-AMPylation by the cognate antitoxin MntA, containing a MNT (minimal nucleotidyltransferase) domain [, , ]. HepT-MnA form an heterooctamer (at a 2:6 ratio), a rare organisation for this kind of TA systems. HepT dimerises and enables the formation of a deep cleft at the HEPN-domain interface, containing the RX4-6H motif (where X is any amino acid and the residue immediately after the conserved R is typically a polar amino acid) as the active site that functions as an RNA-cleaving RNase. This type II TA system regulates cell motility and confers plasmid stability []. Due to the prevalence of these HEPN/MNT modules in bacteria and archaea, it has been suggested that these TA systems may also play a role in the environmental adaptation to extreme habitats []. This family also includes uncharacterised putative RNases from bacteria and archaea.
