The recognition of mismatched base pairs in DNA by DNase I from Ustilago maydis

The activity of Ustilago maydis DNase I, an enzyme implicated in genetic recombination, on DNA substrates containing unpaired or mismatched bases, was examined. The enzyme nicked supercoiled PM-2 molecules, converting these to relaxed circular and linear molecules. Discrete double stranded linear fragments smaller than unit length were also observed after digestion at high enzyme concentration. Heteroduplex molecules were constructed using phi80 bacteriophage derivatives which contained single base substitutions within the E. coli tRNA1tyr gene. Single and double stranded nicking at or near the single mismatched site was observed with three out of the five pairs of heteroduplexes.