Comparisons of ribonuclease HI homologs and mutants uncover a multi-state model for substrate recognition

Ribonuclease HI (RNHI) non-specifically cleaves the RNA strand in RNA:DNA hybrid duplexes in a myriad of biological processes, including retroviral reverse transcription. Several RNHI homologs contain an extended domain, termed the handle region, that is critical to substrate binding. Nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations have suggested a kinetic model in which the handle region can exist in open (substrate-binding competent) or closed (substrate-binding incompetent) states in homologs containing arginine or lysine at position 88 (using sequence numbering of E. coli RNHI), while the handle region populates a state intermediate between the open and closed conformers in homologs with asparagine at residue 88 [Stafford, K. A., et al., PLoS Comput. Biol. 2013, 9, 1-10]. NMR parameters characterizing handle region dynamics are highly correlated with enzymatic activity for RNHI homologs with two-state (open/closed) handle regions [Martin, J. A., et al., Biochemistry 2020, 59, 3201-3205]. The work presented herein shows that homologs with one-state (intermediate) handle regions display distinct structural features compared with their two-state counterparts. Comparisons of RNHI homologs and site-directed mutants with arginine at position 88 support a kinetic model for handle region dynamics that includes 12 unique transitions between eight conformations. Overall, these findings present an example of the structure-function relationships of enzymes and spotlight the use of NMR spectroscopy and MD simulations in uncovering fine details of conformational preferences.