Change of plasmid DNA structure, hypermethylation, and Lon-proteolysis as steps in a replicative cascade.

I have defined conditions under which RepFIC plasmid DNA can be maintained in a state of lowered helical density. In exponentially growing cultures, the DNA of lowered helical density is present in small amounts but never totally absent, suggesting that it is a normal variant of plasmid maintenance. It is fully methylated at frequent sites by dam-methyltransferase, some not previously recognized, further suggesting that the variant is a precursor of replication. The low-helical density plasmid is present in dam hosts, indicating that methylation is not essential for the change in helical density. The lowered helical density is stabilized in lon hosts, suggesting that Lon-protease may remove both the protein(s) that lower the helical density and the dam-methyltransferase after each round of replication.