When packaged in a bacteriophage capsid, double-stranded DNA occupies a cavity whose volume is roughly twice the volume of the DNA double helix. The data thus far have not revealed whether the compactness of packaged bacteriophage DNA is achieved by folding of the DNA, unidirectional winding of the DNA, or a combination of both folding and winding. To assist in discriminating among these possibilities, the present study uses electron microscopy, together with ultraviolet light-induced DNA-DNA cross-linking, to obtain the following information about the conformation of DNA packaged in the comparatively large bacteriophage, G: 1) At the periphery of some negatively stained particles of bacteriophage G, electron microscopy reveals strands of DNA that are both parallel to each other and parallel to the polyhedral bacteriophage G capsid. However, these strands are not visible toward the center of the zone of packaged DNA. 2) Within some positively stained particles, electron microscopy reveals DNA-associated stain in relatively high concentration at corners of the polyhedral bacteriophage G capsid. 3) When cross-linked DNA is expelled from its capsid during preparation for electron microscopy, some DNA molecules consist primarily of a compacted central region, surrounded by DNA strands that appear to be unraveling at multiple positions uniformly distributed around the compacted DNA region. The above results are explained by a previously presented model in which DNA is compacted by folding to form 12 icosahedrally arranged pear- shaped rings.
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