Isopycnography of intact cells - V: The kinetics of the initiation of DNA synthesis in E. coli undergoing nutrient shift-up

Early increases in the frequency of DNA initiations and of phospholipid synthesis discontinuities after nutritional shift-up in Escherichia coli

Cultures of Escherichia coli (strains ML30 and K12 AB1157), synchronized by repeated phosphate starvation, were submitted to nutritional shifts-up at various cell ages. The progression of the replication forks was assessed by DNA-DNA hybridization of pulse-labelled chromosomal DNA with plasmid DNA probes containing specific chromosomal sequences. The rate of phospholipid synthesis and its cyclic discontinuities were measured by continuous and pulse labelling with palmitate. The DNA-DNA hybridization experiments showed that a shift-up induces a burst of initiation from the oriC region. These hybridization results, taken together with older data from the literature, suggest that most DNA initiations belonging to this burst are not followed by complete replication. Following a shift-up, the rate of phospholipid synthesis is maintained for 13-20 min, depending on cell age at shift-up, then doubles. The new steady-state rate of phospholipid synthesis is reached through a series of three doublings, while the cell mass doubles approximately twice. This discrepancy brings the rate of phospholipid synthesis per mass unit to its steady-state postshift value.

Isopycnography of intact cells. VI: Effect of temperature on steady state Escherichia coli

Steady state cultures of Escherichia coli W3110 possess constant cellular concentrations of ribosomes while above 22 C as measured isopycnographically. Below 22 C, ribosome levels are significantly diminished, equivalent to those found in dormant cells. Concommitantly, the energy of activation of steady state population growth in E. coli remains at a constant 18.6 kcal between 22 C and 37 C. This suggests that the thermal stability of the protein synthesis apparatus may play an important role in pathogenesis as well as in other areas of ecological niche acquisition and dominance.

The realm of the steady state in Escherichia coli

Only at cell concentrations of less than 10(7)/ml are E. coli in steady state as defined by three intrinsic properties: mean cell diameter, cellular specific gravity and the concentration of beta-galactosidase per cell. Such concentrations are below the range of turbidimetry.