PROTEIN ENGINEERING OF THE RESTRICTION-ENDONUCLEASE ECORV - REPLACEMENT OF AN AMINO-ACID RESIDUE IN THE DNA-BINDING SITE LEADS TO AN ALTERED SELECTIVITY TOWARDS UNMODIFIED AND MODIFIED SUBSTRATES

According to the crystal structure analysis of a specific EcoRV/DNA co mplex, the thymine residues of the recognition sequence -GATATC- are n ot in direct contact with any amino acid residue of the protein. Howev er, several amino acid residues are sufficiently close that it seemed worthwhile trying to create variants of EcoRV with altered specificity by site-directed mutagenesis. Guided by molecular modelling we have r eplaced Asn-188 in the catalytic center of EcoRV by Gin to produce a m utant with a relative preference (compared to wild type EcoRV) for sub strates in which one thymine of the recognition sequence is replaced b y uracil. We have purified and characterized the resulting N188Q mutan t. The selectivity value for the engineered enzyme (the ratio of the k (cat)/K-M values for -GATAUC- versus -GATATC-) differs from that of th e wild type enzyme by a factor of more than 200.