Cyclic changes of the rate of phospholipid synthesis during synchronous growth of Escherichia coli.

The problem of the coordination between cyclic events in the DNA assembly line and the cell envelope assembly line was approached with the technique of synchronized cultures. Escherichia coli strains ML 30, K12 3300, K12 PC2, K12 BB2014 and B/rF were synchronized by repeated cycles of mass doubling followed by short phosphate starvation periods. Steady-state balanced growth was obtained by subsequent incubation in non-limiting growth conditions for one or more generation times. Several successive cell cycles were monitored for mass increase and cell number, while the rate of DNA synthesis and the rate of phospholipid synthesis were usually measured with more than one method. In all strains, and in strain ML 30 in five different growth media giving doubling times from 20-110 min, a discontinuity in the rate of synthesis of phosphatidylethanolamine and of phosphatidylglycerol was observed. These two major phospholipid components of inner and outer membranes were synthesized at a constant rate per cell for a large portion of the cell cycle and the rate of synthesis of both increased twofold at the same time. This cyclic program was reproducible not only in successive cell cycles, but also in separate experiments with the same strain, in the same medium. In contrast, differences in timing were observed with different strains, and in the same strain with different carbon sources. In particular, the simultaneity of the increase in phospholipid synthesis either with DNA initiation or with cell division could not be observed as a rule.