The substrate spectrum of Vsr DNA mismatch endonuclease ofEscherichia coliK12 was investigated using fluorescence-labelled oligonucleotide substrates and a DNA sequencer for detection and quantification of substrates and reaction products. Fourteen substrates were found to be processed by the enzyme, which differ in one or two positions from the canonical pentanucleotide sequence CT A / T GG(Tmismatched to G). Relative second-order rate constants of these substrates were determined in groups of four by multiple substrate kinetics and compared to the underrepresent ation of the corresponding pentanucleotides in theE. coliK-12 genome. The high quality of correlation further establishes active mutagenesis by VSP repair as a significant driving force of the evolution of theE. coliK-12 genome and provides clues to its possible selective value.