On the possibility of obtaining a physical map of genomes by photoelectron imaging.

Photoelectron imaging provides the possibility of a new method of mapping chromosomes. The basic concept is to cause DNA to emit electrons under the action of UV light. The criteria which must be met to map genomes by photoelectron imaging are set forth and discussed. Forming an image of the DNA by accelerating and focusing the electrons is a necessary but not sufficient condition for genome mapping. Equally important is to identify wavelengths of UV light which will cause selective emission from the base pairs, adenine-thymine and guanine-cytosine. The resulting image would then contain a modulation in the image brightness along the DNA duplex. By examining the photoelectron current from uniform films of homopolymers, a wavelength region is identified where marked differences in emission from base pairs is observed. At 160 nm, for example, the relative electron emission from a film of poly(dGdC) is approximately 5 times greater than for an equivalent film of poly(dAdT). Using the experimental data and known sequences, photoelectron gene maps are calculated for the bacteriophage lambda and for a short interspersed repetitive DNA sequence (an Alu repeat) of the human genome. The results suggest that a 5-nm physical map of chromosomes generated by photoelectron imaging would be informative and useful in mapping human and other large genomes.