The sedimentation rate of bacteriophage T7 DNA from T7-infected Escherichia coli has been determined after lysis with the nonionic detergent Brij 58. After 8.5 min of infection at 37°, most of newly replicated T7 DNA sedimented more rapidly than 100 S(100 S+ DNA). Some of the T7 100 S+ DNA sedimented in a broad peak between 100 Sand 600 S; the remainder sedimented at speeds greater than 600 S. In the electron microscope 100 S+ T7 DNA appeared to be a massive DNA complex containing up to 30 phage equivalents of DNA in a single structure. In these complexes densely packed cores of DNA were surrounded by more loosely packed DNA. Kinetic labeling experiments strongly suggest that 100 S+ DNA is a precursor for DNA in progeny phage. It was also shown that: (i) Non-DNA substances, possibly membrane fragments and/or proteins, are bound to 100 S+ T7 DNA, but are probably not the primary cause of the rapid sedimentation of this DNA. (ii) Single-chain interruptions exist in the predominantly duplex structure of T7 100 S+ DNA. (iii) When cells are lysed with the ionic detergent, Sarkosyl, the 100 S+ DNA is obtained, though in reduced yield. Phenol extraction eliminates virtually all 100 S+ DNA. (iv) Some of the 100 S+ T7 DNA is more shear-resistant than linear, duplex DNA.
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