THE INITIATION MASS FOR DNA-REPLICATION IN ESCHERICHIA-COLI K-12 IS DEPENDENT ON GROWTH-RATE

It is widely accepted that the initiation mass of Escherichia coli is constant and independent of growth rate, and therefore is an important parameter in the regulation of initiation of DNA replication. We have used flow cytometry to measure the initiation mass of E.coli K-12 cel ls as a function of growth rate. The average initiation mass was deter mined by two methods: (i) from a mathematical relationship between ave rage cell mass, cell age at initiation and number of origins present i n the cells, and (ii) directly from the cell mass distribution. The li ght scattering signal from individual cells and the protein content pe r cell were employed as measures of cell mass. The initiation mass was found to increase monotonically with decreasing growth rate, being 1. 6 times higher (light scattering) or 2.1 times higher (protein content ) at 0.3 than at 2.5 doublings per hour. We conclude that the initiati on mass is dependent on growth rate. This finding indicates that the c ontrol for timing of initiation is not governed by a direct connection between mass accumulation and the molecule(s) determining initiation of replication.