Complexes of capsids with monomeric bacteriophage T7 DNA and complexes of capsids with concatemeric bacteriophage T7 DNA have been isolated from lysates of T7-infectedEscherichia coli and the capsids of these complexes have been characterized. In electron micrographs capsids bound to either monomeric DNA or concatemeric DNA have envelopes which are: (1) significantly less thick than the envelope of a DNA-free capsid (capsid I) isolated from T7-infectedE. coli, (2) indistinguishable from the envelope of bacteriophage T7 and the envelope of a second DNA-free capsid (capsid II) isolated from T7-infectedE. coli. By agarose gel electrophoresis (under nondenaturing conditions) and SDS-polyacrylamide gel electrophoresis the capsids released from the above capsid-DNA complexes are similar to capsid II and different from capsid I. The physical characteristics and radiolabeling kinetics of these capsid-DNA complexes suggest that they are DNA packaging intermediates; the results suggest a pathway for the packaging of DNA by bacteriophage T7.
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