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.