Potassium transport inEscherichia coli: sodium is not a substrate of the potassium uptake system TrkA

Change to alanine of one out of four selectivity filter glycines in KtrB causes a two orders of magnitude decrease in the affinities for both K+ and Na+ of the Na+ dependent K+ uptake system KtrAB from Vibrio alginolyticus

KtrAB from Vibrio alginolyticus is a recently described new type of high affinity bacterial K+ uptake system. Its activity assayed in an Escherichia coli K+ uptake negative mutant depended on Na+ ions (Km of 40 microM). Subunit KtrB contains four putative P-loops. The selectivity filter from each P-loop contains a conserved glycine residue. Residue Gly-290 from the third P-loop selectivity filter in KtrB was exchanged for Ala, Ser or Asp. KtrB variants Ser-290 and Asp-290 were without activity. In contrast, KtrB variant Ala-290 was still active. This variant transported K+ with a two orders of magnitude decrease in apparent affinity for both K+ and Na+ with little effect on Vmax.

Cloning and sequencing of a K+ transport gene (trk A) from the marine bacterium Vibrio alginolyticus

A gene has been cloned from the marine bacterium Vibrio alginolyticus that functionally complements a mutant strain of Escherichia coli, TK420, defective in K+ transport genes (kdpABC, trkD, trkA). The cloned Vibrio gene allowed TK420 to grow in a synthetic medium containing less than 10 mM K+ and concomitantly led to an increase in K+ uptake activity. The nucleotide sequence of the cloned fragment revealed an open reading frame, which encodes a protein with a predicted 458 amino acid sequence and molecular mass of 50,122 Da. This gene has 71% homology to trkA gene at the DNA level from E. coli and the deduced amino acid sequence is 79% identical with E. coli TrkA, implying that V. alginolyticus has a trkA-like gene as a component of K+ transport systems.

Relationship between the F0F1-ATPase and the K(+)-transport system within the membrane of anaerobically grown Escherichia coli. N,N'-dicyclohexylcarbodiimide-sensitive ATPase activity in mutants with defects in K(+)-transport

A considerable (2-fold) stimulation of the DCCD-sensitive ATPase activity by K+ or Rb+, but not by Na+, over the range of zero to 100 mM was shown in the isolated membranes of E. coli grown anaerobically in the presence of glucose. This effect was observed only in parent and in the trkG, but not in the trkA, trkE, or trkH mutants. The trkG or the trkH mutant with an unc deletion had a residual ATPase activity not sensitive to DCCD. A stimulation of the DCCD-sensitive ATPase activity by K+ was absent in the membranes from bacteria grown anaerobically in the presence of sodium nitrate. Growth of the trkG, but not of other trk mutants, in the medium with moderate K+ activity did not depend on K+ concentration. Under upshock, K+ accumulation was essentially higher in the trkG mutant than in the other trk mutant. The K(+)-stimulated DCCD-sensitive ATPase activity in the membranes isolated from anaerobically grown E. coli has been shown to depend absolutely on both the F0F1 and the Trk system and can be explained by a direct interaction between these transport systems within the membrane of anaerobically grown bacteria with the formation of a single supercomplex functioning as a H(+)-K+ pump. The trkG gene is most probably not functional in anaerobically grown bacteria.

Specific cesium transport via the Escherichia coli Kup (TrkD) K+ uptake system

Escherichia coli cells which contain a functional Kup (formerly TrkD) system took up Cs+ with a moderate rate and affinity. Kup is a separate K+ uptake system with relatively little discrimination in the transport of the cations K+, Rb+, and Cs+. Regardless of the presence or absence of Kup, K+-replete cells took up Cs+ primarily by a very low affinity mode, proportional to the ratio of the Cs+ and K+ concentrations in the medium.

Low-affinity potassium uptake system in Bacillus acidocaldarius

Cells of Bacillus acidocaldarius that were grown with 2.7 mM K+ expressed a low-affinity K+ uptake system. The following observations indicate that its properties closely resemble those of the Escherichia coli Trk and Streptococcus faecalis KtrI systems: (i) the B. acidocaldarius system took up K+ with a Km of 1 mM; (ii) it accepted Rb+ (Km of 6 mM; same Vmax as for K+); (iii) it was still active in the presence of low concentrations of sodium; (iv) the observed accumulation ratio of K+ maintained by metabolizing cells was consistent with K+ being taken up via a K+-H+ symporter; and (v) K+ uptake did not occur in cells in which the ATP level was low. Under the latter conditions, the cells still took up methylammonium ions via a system that was derepressed by growth with low levels of ammonium ions, indicating that in the acidophile ammonium (methylammonium) uptake requires a high transmembrane proton motive force rather than ATP.

Physical mapping of the K+ transport trkA gene of Escherichia coli and overproduction of the TrkA protein

The position on the Escherichia coli chromosome of trkA, a gene coding for a membrane protein involved in K+ transport by the constitutive uptake system Trk, was determined. We observed that the gene is transcribed in a clockwise direction and that it is located at 72.4 min on the chromosome in a 1.75-kilobase NruI-EcoRV DNA fragment 1.0 kilobase upstream of rplQ. We localized an additional gene encoding a 17,000-molecular-weight protein of unknown function between the trkA and rplQ genes. A plasmid, pDB3, was constructed in which the transcription of the trkA gene was put under the control of the lambda pL promoter. pDB3-containing cells of a strain, which contained the temperature-sensitive lambda repressor cI857 in the chromosome, overproduced the 53,000-molecular-weight TrkA protein at the nonpermissive temperature to such an extent that TrkA became the major cell protein. From cell fractionation studies, we conclude that the overproduced TrkA protein forms aggregates.