Folded, concatenated genomes as replication intermediates of bacteriophage T7 DNA

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Abstract
A complex form of bacteriophage T7 DNA, containing up to several hundred phage equivalents of DNA, arises during T7 replication. The complex was stable to treatment with ionic detergent, Pronase and phenol. The complex form normally exists for only a short time, corresponding to the phase of rapid T7 DNA synthesis. It is then converted to shorter molecules, both concatemers and unit-size DNA. The complex was stable up to the denaturation temperature of the bihelix. It consisted of a series of loops emanating from a dense central core, as shown by EM. The complex form is similar to the relaxed Escherichia coli folded chromosome (nucleoid). The loops contained an average of 0.7-0.8 phage equivalent of DNA. During infection by phage with an amber mutation in gene 3 (endonuclease), formation of the complex occurred normally, but its maturation to unit-size DNA was blocked. Before treatment with phenol, the complex contained short fragments of newly replicated DNA. These were released as single-stranded pieces during phenol treatment. A pathway for T7 DNA replication is indicated in which the flow of material is from unit-size DNA to linear concatemers to the complex form, and then back to unit-size DNA by way of linear concatemers.