Rapid response to osmotic upshift by osmoregulated genes in Escherichia coli and Salmonella typhimurium.

The rapid in vivo response of both Escherichia coli and Salmonella typhimurium osmoregulated genes to an osmotic upshift was analyzed in detail by using chromosomal operon fusions. Within 10 min after the addition of 0.3 M NaCl to the culture medium, the differential rates of expression of both an S. typhimurium proU-lac fusion and a proP-lac fusion increased by 180- and 17-fold respectively, while an E. coli ompC-lac fusion increased by 3.4-fold. For all three stimulated promoters, the increased rate of expression was maintained until about 40 min after the osmotic upshift. Thereafter, proU expression continued at a steady-state rate that was 27-fold higher than that of the control, while proP and ompC expression fell to 1.4- and 2-fold of the control rates, respectively. In contrast, expression of an E. coli ompF-lac fusion decreased twofold within 2.5 min. For proU, the length of the lag phase, which preceded the onset of the rapid response, increased with the degree of osmotic upshift, above a threshold of 0.2 M NaCl; the onset of the rapid proU response also preceded the resumption of growth. The rapid response phase, which was first quantitated for proU, proP, ompC, and ompF in this study, is an important component of the osmoregulation of these promoters. The addition of the osmoprotectant glycine betaine at the time of osmotic upshift decreased both the length of the rapid response and the subsequent steady-state of expression of proU.