Mapping Individual Cosmid DNAs by Direct AFM Imaging

Individual cosmid clones have been restriction mapped by directly imaging, with the atomic force microscope (AFM), a mutantEcoRI endonuclease site-specifically bound to DNA. Images and data are presented that locate six restriction sites, predicted from gel electrophoresis, on a 35-kb cosmid isolated from mouse chromosome 7. Measured distances between endonuclease molecules bound to λ DNA, when compared to known values, demonstrate the accuracy of AFM mapping to better than 1%. These results may be extended to identify other important site-specific protein–DNA interactions, such as transcription factor and mismatch repair enzyme binding, difficult to resolve by current techniques.