Site-directed mutagenesis in the catalytic center of the restriction endonuclease EcoRI

Abstract

The catalytic center of the restriction endonuclease (ENase) EcoRI is structurally homologous to that of EcoRV, BamHI and PvuII. Each of these ENases contains a short motif of three to four amino acid (aa) residues which are positioned in a similar orientation to the scissile phosphodiester bond. We have mutated these aa (Pro90, Asp91, Glu111 and Lys113) in EcoRI to determine their individual roles in catalysis. The replacement of Asp91 and Lys113, respectively, by conservative mutations (Ala91, Asn91, Ala113, Gln113, His113 and Leu113) resulted in a reduction of binding affinity and complete loss of cleavage activity. Only Lys113-->Arg substitution still allows to cleave DNA, albeit with a rate reduced by at least four orders of magnitude. Lys113 seems to stabilize the structure of the wild-type (wt) ENase since all five ENase variants with mutations at this position show a strongly enhanced tendency to aggregate. The Ala and Gln mutants of Glu111 bind the recognition sequence slightly stronger than wt EcoRI and cleave it with a low, but detectable rate. Only the Glu111-->Lys mutant, in which the charge is reversed, shows neither binding nor cleavage activity. Pro90 is not important for catalysis, because the Ala90 mutant cleaves DNA with an only slightly reduced rate. Under star conditions, however, this mutant is even more active than wt EcoRI. Therefore, the charged aa Asp91, Glu111 and Lys113 are essential for catalytic activity of the EcoRI ENase.(ABSTRACT TRUNCATED AT 250 WORDS)