Identification of osmoresponsive genes in Escherichia coli: evidence for participation of potassium and proline transport systems in osmoregulation

Abstract

Mu dl(Ap lac)-generated operon fusions were used in the identification of genes in Escherichia coli whose transcriptional expression is altered by changes in the osmolarity of the growth medium. One such osmoresponsive gene, designated osrA, was induced 400-fold when the osmolarity of the medium was increased with the addition of either ionic or neutral impermeable solutes but was not induced with glycerol, which is freely permeable across the cell membrane. osrA was mapped to 57.5 min and was shown to be transcribed clockwise on the E. coli chromosome. The ability of small concentrations of L-proline to promote the growth of E. coli in high-osmolar medium was shown to have been specifically lost in osrA mutants; other lines of evidence were also obtained to support the notion that osrA codes for an osmoresponsive L-proline transport system and is homologus to proU in Salmonella typhimurium. A second osmoresponsive operon identified was kdp, which codes for an inducible K+-transport system in E. coli. kdp expression was elevated 12-fold when the osmolarity of the growth medium was increased with the addition of impermeable ionic solutes but not neutral solutes; furthermore, osmoresponsivity of kdp expression was demonstrable only in K+-limiting media. kdp mutants were able to grow normally in high-osmolar media, but strains defective in both kdp and trkA (a gene for a second major K-transport system) displayed an osmosensitive phenotype. The results suggest that transport systems for L-proline and K+, specified by osrA (proU) and kdp, respectively, play independent and important roles in osmoregulation in E. coli. A third osmoresponsive gene that was identified was lamB, which codes for an outer membrane protein for maltodextrin transport and A phage adsorption; its expression was reduced fourfold with increase in the osmolarity of the growth medium. 4 6 35 15 28 B. J. Bachmann B. J. Bachmann S. K. Mahajan 43 This study This study This study This study This study This study This study This study This study This study This study This study This study This study This study This study This study From KL14 (28), by P1 kc transduction This study From GJ11, by P1 kc transduction From GS74, in two steps From CSH57, in two steps This study From GJ11, by P1 kc transduction to Put-From MC4100, by P1 kc transduction to Put-From GJ113, by P1 kc transduction to Put-DHPF selection from Tets derivative of GJ125 DHPI selection from Tets derivative of GJ126 DHPI selection from Tets derivative of GJ121 By conjugation, KL584 x GJ134 zjd-351::TnIO By conjugation, KL584 x GJ135 zjd-351::TnlO By conjugation, KL584 x GJ157 a The nomenclature for genetic symbols follows that described by Bachmann (1), and the nomenclature for transpositional insertions follows that described by Chumley et al. (9). Allele numbers are indicated if they are known.

The complexity of the ‘simple’ two-component system KdpD/KdpE inEscherichia coli

“…Because K 1 plays a major role in maintaining turgor, it was supposed that KdpD senses a decrease in turgor or some effect thereof (Laimins et al, 1981;Malli & Epstein, 1998). This model has been challenged by the finding that kdpFABC expression is only induced when the osmolarity is increased by a salt and not by a sugar (Gowrishankar, 1985;Sutherland et al, 1986;Asha & Gowrishankar, 1993). Therefore, Mizuno and colleagues suggested that the sensing mechanisms for K 1 limitation and osmotic upshift are mechanistically different (Sugiura et al, 1994).…”

Osmoregulation and compatible solutes in eubacteria

“…It is most probably energized by the proton motive force and regulated by phosphorylation [39][40][41]. The Kdp system is not dependent on the proton motive force, but represents a potassium-stimulated ATPase, which is repressible and has a very high affinity for potassium (K m = 2/xM) [42][43][44][45]. Under most conditions, potassium is taken up by the Trk system.…”

“…Under most conditions, potassium is taken up by the Trk system. Activities of both systems are controlled by the turgor pressure, the control probably being mediated by a single mediator interacting with both systems [39,45]. It has been concluded that the potassium concentration does not cause expression of the Kdp system, but that the turgor pressure regulates the Kdp operon [42,44,46].…”

“…Growth of Salmonella oranienburg occurred at much higher osmolarities in complex media as compared to simple glucose mineral media, and much of this effect was ascribed to several amino acids in the medium, proline being regarded as the most important one [65]. The intracellular proline level was elevated in direct proportion to increases of the osmolarity of the medium, when proline was present [5,7,45,63,64,68]. In all Enterobacteria tested so far, accumulation of proline is dependent on uptake from the medium, except for some overproducing mutants [8,66,[69][70][71].…”

DNA twist, flexibility and transcription of the osmoregulated proU promoter of Salmonella typhimurium.

“…proU provides the paradigm for a promoter regulated by environmentally induced changes in DNA topology. TheproU operons of Salmonella typhimurium and Escheri-chia coli encode transport systems for the osmoprotectant glycine betaine, and transcription from the proU promoter is increased up to 100-fold by increased extracellular osmolarity (Caimey et al, 1985;Dunlap and Csonka, 1985; Gowrishankar, 1985;May et al, 1986). Although the mechanisms of osmoregulation have yet to be elucidated fully, transcription from the proU promoter is sensitive to factors which perturb DNA topology Ni Bhriain et al, 1989;Hulton et al, 1990).…”