Molee. gen. Genet. 138, 143--155 (1975) 9 by Springer-Verlag 1975 Independence of F Replication and Chromosome Replication in Escherichia coli R. H. Pritchard, M. G. Chandler, and J. Collins Department of Genetics, The University, Leicester, England l~eceived November 15, 1974 Summary. Data are presented which show that F replication is not coupled to any stage of the replication cycle of the host chromosome or to cell division, and is probably not related to surface area. It is also shown that the initiation mass of F increases progressively as the growth rate increases, the number of copies of F per unit of mass falling by half between doubling times of 0.8 and 2.7 generations per hour. It is further shown that the presence of an F particle does not influence the initiation mass of the chromosome. 1. Introduction Current evidence is consistent with the hypothesis that in Escherichia coli the initiation mass (the cell mass per chromosome origin at the time of initiation of rounds of chromosome replication) is constant over a broad range of growth rates. This relationship ensures that the frequency of new rounds of replication and the frequency with which the culture mass doubles will be identical. In a formal sense it constitutes a mechanism for the control of DNA synthesis (see Pritchard, Barth and Collins, 1969). Less is known about the way in which replication of extrachromosomal particles is matched to the growth rate of the cell. It seems clear, however, that in the case of such particles as the sex factor F (the number of which per cell is quite small but whose inheritance is relatively stable) a mechanism must exist which couples the frequency of its replication to the cell growth rate. The ex- periments that will be reported in this paper are concerned with the nature of this mechanism. Several groups (Zeuthen and Pato, 1971; Cooper, 1972; Davis and Helm- stetter, 1973) have attempted to determine the time in the cell cycle at which F replicates in E. coli B/r using the elution technique of ttelmstetter and Cum- mings (1963, 1964). The strain carried a deletion of the chromosomal lac gene and an F-lac+ particle, the time of replication of F being determined from the time in the cell cycle at which the potentiality to synthesise fl-galactosidase doubled. The data of Cooper (1972) are difficult to evaluate because the poten- tiality to synthesise the enzyme was measured during a shift-up from a poor to a rich medium and the influence of changes in the degree of eatabolite repression were not considered. Both Zeuthen and Pato (1971) and Davis and Helmstetter (1973) concluded that at slow growth rates F replication and initiation of chromosome replication occur at different times in the cell cycle suggesting that the two events are not coupled. At growth rates greater than 1.2 doublings per