MAPPING OF DELETIONS AND SUBSTITUTIONS IN HETERODUPLEX DNA MOLECULES OF BACTERIOPHAGE LAMBDA BY ELECTRON MICROSCOPY

Electron microscopy of heteroduplex DNA molecules, composed of one strand of Escherichia coli phage λ+ DNA annealed to the complementary DNA strand of a λ deletion or substitution mutant, permits visualization, as well as precise measurements and mapping, of the unpaired single-stranded regions of nonhomology in the otherwise double-stranded molecules. In the λb2 mutant, the central segment (13 percent) of the λ+ DNA molecule is shown to be deleted. In the hybrid phages λi434 and λi21 a segment of the right arm of the λ+ genome (5.5 or 7.6 to 9 percent) is replaced by the corresponding immunity regions of phage 434 (3.3 percent or phage 21 (4 percent) DNA. The b5 region in the λb5 mutant appears to be identical to the i21 segment. From these data it is possible to estimate the size and posiion of those λ genes which are replaced by the i434 and i21 segments. The method permits preparing complete physical maps of viral genomes with a precision heretofore unattainable.