Mutagenesis of Escherichia coli uridine phosphorylase by random pentapeptide insertions

Escherichia coli uridine phosphorylase (UP) is encoded by the udp gene and catalyzes the reversible phosphorolysis of uridine to uracil and ribose-1-phosphate. Only few key residues involved in UP catalysis, identified by site-directed mutagenesis and selective chemical modification studies, were reported. A recent paper reporting the crystal structure at 2.0 Å resolution indicated that UP shares a high structural homology with E. coli purine nucleoside phosphorylase. This latter enzyme is better known and a number of residues in its active site have been identified. In this work, we used pentapeptide scanning mutagenesis to cause random insertions of a 5 amino acid cassette in the UP polypeptide chain. Several insertional mutants located in different regions of UP maintained or increased the enzymatic activity and provided new insights into protein structure–function relationships. Moreover, this mutagenesis approach appears to be useful for the rapid preparation of mutants that present altered enzymatic activities.