Capsid-DNA complexes in the DNA packaging pathway of bacteriophage T7: Characterization of the capsids bound to monomeric and concatemeric DNA

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.