Domains are colored as described in Figure 1. Select secondary structural elements are numbered for reference. The NotI dimer interface is formed by the domain-swapped alpha helices (cyan) and the DNA recognition helices (red).

Superposition of apo (blue) and DNA-bound (orange) NotI structures for visualization of the conformational change which takes place upon binding of DNA. A decrease in distance between iron atoms of approximately 5.7Å is observed between the two structures, illustrating the motion taking place in the metal-binding and clamp domains. Less movement is observed in the nuclease core and dimerization regions of the structure. The DNA from the DNA-bound structure is colored white and shown for reference.

Panel A: Active site residues Glu145, Asp160, Glu182, and Gln184 are colored yellow. The bound DNA is colored white with the scissile phosphate designated orange. The nucleophilic water molecule is shown as a blue sphere and two calcium ions are depicted as magenta spheres. Dashed lines represent interactions between atoms in the active site and numbers indicate distances in angstroms (as average values calculated between the two active sites of the NotI homodimer). Panel B: Superposition of active site residues of the PD…(D/E)xK active site motif from NotI (green) and BglII (orange). Both REases have a glutamine in the general base position typically occupied by a lysine residue. Panel C: Superposition of the two-metal ion active sites of NotI (green) and BamHI (purple). Asp160/Asp94, Glu182/Glu111, and Gln184/Glu113 are residues belonging to the canonical PD…(D/E)xK nuclease motif, while the 3^rd acidic residue, Glu145/Glu77, helps coordinate a second metal ion in the active site. The position of this 3^rd acidic residue is conserved between the NotI and BamHI enzymes.

The protein is colored according to individual structural domains and motifs; the same color scheme is used in all panels. The N-terminal 85 residues (blue) form a unique metal-binding domain that uses a four-cysteine coordination scheme; this domain is followed by the PD…D/E-xK catalytic domain (green). An elaboration on this fold, extending from between two β-stands, presents a two-helix bundle “clamp” domain (purple) that wraps around the metal-bound domain. This region, along with an adjacent recognition helix (red) contributes the majority of direct contacts between the protein and bound DNA. Finally, two helices extend from the C-terminal end of the protein and form a domain-swapped dimerization motif (cyan). Additional structural elaborations on the nuclease core are colored gray. Panel A: A single monomer of NotI with the full-length (22bp) palindromic DNA target site. The bound iron ion is shown as an orange sphere, and select secondary structural elements are labeled by number for reference. Panel B: The homodimeric endonuclease bound to its cognate DNA target site. One subunit is colored as described above (the opposite protein chain colored relative to panel a); the second subunit is colored uniformly dark for clarity and contrast. The cysteine sidechains of the Fe-Cys₄ iron-binding motif are shown as sticks and colored by element. Panel C: Topology of the NotI protein fold. Residues at boundaries of secondary structural elements are numbered; active site residues are indicated with magenta stars and bold labels; The bound iron is shown as an orange circle, and iron ligands (Cys 42, 55, 65, and 81) are represented by green hexagons. Individual secondary structural elements are numbered in sequential order. Panel D: Sequence homology between NotI, EagI (GenBank EU 371940, which recognizes and cuts a six-base site corresponding to the central six positions of the 8-base NotI target site), and homologues ZP_01014694.1 from Rhodobacterales bacterium HTCC2654, ZP_01998632.1 from Beggiatoa sp. PS, and YP_001440430.1 from Enterobacter sakazakii ATCC BAA_894. Iron-binding residues (of which three out of four are conserved in EagI) are indicated with circles (o); DNA-binding residues (of which ten out of nineteen are conserved in EagI) are indicated with asterisks (*); and catalytic residues of the PD…(D/E)xK endonuclease motif (two-metal ion mechanism) are indicated with diamonds (◊) (all are conserved).

Panel A: FeCys₄ Proteins - Rubredoxin (6rxn), desulforedoxin (1dxg), and rubrerythrin (1ryt) are three redox enzymes which contain the Fe-Cys₄ iron binding motif, often referred to as the “rubredoxin-type” fold. Panel B: Homodimeric view of NotI with metal-binding domains highlighted in blue. Bound irons are shown as orange spheres and coordinating cysteine residues are colored by element and represented as sticks. The metal-binding domain is sandwiched between the nuclease core of the protein and the clamp domain (see panel c). Panel C: Iron-binding domain (blue) flanked by the clamp domain (purple). The iron-binding domain appears to serve an essential structural role in positioning of the clamp domain for contact with the DNA. The base of the DNA recognition helix making additional contacts to the DNA is also shown (red).

Panel A: Contacts make by the NotI restriction endonuclease to its 8 basepair DNA recognition sequence. Orange circles represent the phosphate groups of the DNA sugar-phosphate backbone. The scissile phosphats are colored red. Mounds and indents on the cartoon blocks represent the hydrogen bond donors and acceptors, respectively, of the individual nucleotide bases. The right hand side of the bases represents the major groove of the DNA and the left side represents the minor groove. The 2-fold rotation axis in the center of the cartoon designates the center of symmetry for the palindromic target DNA sequence. Small blue circles represent water molecules, and magenta circles are calcium ions. Residues from the active site are colored red. The site of DNA cleavage is designated by a dashed red line. The bases of the recognition sequence are numbered both outward from the center of symmetry (designated Site #) as well as by their PDB ID from the model coordinates. Panel B: Specific contacts made to each basepair of the NotI recognition sequence. Protein sidechain stick representations are colored based on their domain of origin: clamp domain (purple) or DNA recognition helix (red). Important atoms of bases and sidechains are colored by element. Hydrogen bonding interactions are represented by dashed lines. Waters are shown as blue spheres.

A variety of studies, described in the text, indicate that restriction endonucleases may have originated from ancestral invasive endonucleases (and in fact may still display features of mobility and 'selfishness'. The original invasive mobile endonucleases would be required to display long target site recognition, in order to avoid toxicity to the host; the persistance of such elements would generally be opposed by pressure to minimize and streamline genome size. The acquisition of a beneficial secondary activity would improve the fitness of the host in the presence of the endonuclease gene. One such mechanism for the development of a beneficial function would be the coupling of a DNA modification activity that blocks the cleavage activity of the endonuclease in the host, with subsequent reduction in its cleavage specificity. If invasive endonuclease families (such as homing endonucleases) were related to defensive restriction endonucleases, one would expect to detect the presence of restriction endonucleases with nonoptimal, overly long target sites that are obviously related to more 'advanced' short site cutters. The NotI endonuclease (with its reduced contacts to the basepairs at positions +/−4) and its relationship to the six-basepair cutter EagI appears to fulfill the requirements of an invasive/defensive restriction endonuclease 'intermediate'.