Experimental and Theoretical Considerations of P1-plasmid Replication and Segregation During the E. coli Cell Cycle

2 Abstract: Two contrasting experimental descriptions of P1 plasmid replication during the cell cycle of Escherichia coli have been described. One set of results led to the proposal that replication of P1 plasmid occurs at a specific time during the cell cycle over a wide range of growth rates and follows rules similar to that governing bacterial chromosome replication. Experiments supporting this proposal utilized membrane-elution experiments, radioactive double-labeling of DNA and scintillation counting of purified plasmids. An alternative experimental description of P1 plasmid replication during the cell cycle, also based on membrane-elut ion methodology but measuring radioactivity incorporated into plasmid DNA by autoradiograp hy and scanning of films, proposed that P1 plasmid replicates at all stages of the cell cycle in rapidly growing cells, but with a slight periodicity or increase in P1 replication probability within the cell cycle of slower growing cells. These discordant experimental results are analyzed. It is concluded that the direct double-label counting approach is to be preferred, as the results are consistent with a large number of experiments, are supported by theoretical considerations and yield a unified view of plasmid replication over a wide range of growth rates. Theoretical ramifications of each view of P1 plasmid replication-cycle-dependent and cycle-independent are compared. An analysis of P1 plasmid segregation is also presented.