Nonintegrated plasmid-folded chromosome complexes: genetic studies on formation and possible relationship to plasmid replication.

When folded chromosomes are purified from plasmid-containing bacteria, a reproducible fraction of the host's covalently closed, circular (CCC) plasmid DNA copurifies with the chromosomes. From this copurification, we infer the existence of nonintegrative plasmid-chromosome (NPC) complexes. Previously, we noted that plasmids dependent on DNA polymerase III and with stringent control of replication complex to a greater extent than plasmids dependent on DNA polymerase I and with relaxed control of replication. We have examined this subject in more depth and find that: (i) The composite plasmids formed by in vitro recombination of a “stringent” with a “relaxed” replicon complex to chromosomes at the frequency of the component replicon which directs replication; (ii) all of the detectable replicative intermediates, but only 25% of the CCC forms, of plasmid ColE1 complex to chromosome; and (iii) when a mini-F plasmid is deleted for the DNA sequences which include the primary origin of replication, the complexing frequency decreases 30 to 40%. We conclude from these findings that NPC complexes either indirectly or directly relate to plasmid replication. Further, we find that the EcoRI kan⁺ fragments of pML31 and the ampicillin resistance transposon, Tn3, promote complexing of both ColE1 and mini-F plasmids to host chromosomes. The biological significance of this latter complexing is unknown. However, we conclude from these studies and from point (iii) that complexing is determined in part by unique plasmid sequences.