SATURATION MUTAGENESIS OF HIS(114) OF ECORI REVEALS RELAXED-SPECIFICITY MUTANTS

EcoRI recognizes and cleaves DNA at GAATTC sites and is one of the best characterized sequence-specific restriction endonucleases (ENases). In previous studies, an EcoRI mutant, which exhibited relaxed substrate specificity and cleaved both canonical and EcoRI star sites, was isolated. This mutant enzyme has Tyr instead of His(114). Here, we subjected residue 114 of the EcoRI ENase to saturation mutagenesis. The resulting mutant enzymes were characterized both in vivo and in vitro, resulting in the identification of mutants with canonical (H114K, Q, D, I) or relaxed (K114Y, F, S, T) specificity, as well as one mutant with severely impaired activity (H114P). In the X-ray structure of an EcoRIsubstrate complex, His(114) is located between the catalytic and recognition regions of EcoRI and may directly contact the DNA phosphate backbone. Based on our genetic and biochemical findings and the X-ray structure, we propose that His(114) participates in substrate recognition and catalysis, either directly, via protein-DNA interactions, or indirectly, by mediating conformational changes that trigger DNA cleavage in response to substrate recognition.