KeywordsEPR spectroscopy; DEER; DNA binding protein; structure elucidation; protein flexibility; noncognate complexIn this communication, we show that the EcoRI restriction endonuclease binds different classes of DNA sites in the same binding cleft. EcoRI generates widespread interest because it exhibits an extraordinary sequence selectivity to carry out its function of cleaving incoming foreign DNA without causing potentially lethal cleavage of cellular DNA. For example, EcoRI binds to its correct recognition site GAATTC up to 90,000-fold better than to miscognate sites that have one incorrect base pair. [1,2] The ∼650 specific sites in the E. coli genome are protected from cleavage by double-strand methylation. The ∼21,000 miscognate sites are not methylated, but are still cleaved by EcoRI with a second-order rate constant that is ∼10 9 -fold lower.[1,2] EcoRI forms only non-specific complexes with no cleavage at sites that differ from GAATTC by two or more base pairs. [1,2] In order to understand the source of such high specificity, it is necessary to determine how the structures of EcoRI complexes differ at specific, miscognate (5/6 bp match), and nonspecific (≤4/6 bp match) DNA sites. This effort is timely given the extensive genetic, biochemical and biophysical data on EcoRI.[1-9] Footprinting results [1] suggest that the three classes of complexes are "structurally" distinct, and thermodynamic profiles (ΔG°, ΔH°, ΔS°, ΔC°P) [3,4] suggest that the specific complex has more restricted conformationalvibrational mobility of the protein and DNA. There are crystal structures of the free protein, Figure 1 shows the structure of the EcoRI specific complex. [6,7] The protein contains a large, relatively rigid and structured globular "main" domain and a smaller "arm" region. The protein arms are invisible in the free protein[6] but become ordered and enfold the DNA in the specific complex, where they play a role in modulating specificity.[2,4] Mutations R131C, S180C, and K249C-S180C were chosen based on the crystal structure. [6,7] These sites are solvent accessible and therefore likely to spin label with minimal perturbation to protein structure. Residues R131 and S180 lie in the inner and outer arms, respectively. Residue K249 is in the main domain, which has very restricted movement [6] and acts as a reference point. Since EcoRI is a 62 kDa homodimer, single cysteine mutations provide two sites for spin labeling, and double mutations provide four sites.The proteins were spin labeled at the cysteines with the methanethiosulfonate spin label (MTSSL). There is an intrinsic cysteine at position 218, but it is buried, leading to <10% labeling even with a 100-fold molar excess of the spin label. The mutant proteins and their spin labeled derivatives catalyze DNA cleavage and have DNA binding affinities similar to that of wild type EcoRI, indicating that they are functionally active (Supporting Information).DEER experiments [11] were performed on spin labeled S180C specific and non-specific complexes, and on R131C and...