KtrAB and KtrCD: Two K⁺Uptake Systems inBacillus subtilisand Their Role in Adaptation to Hypertonicity

Abstract

Recently, a new type of K ؉ transporter, Ktr, has been identified in the bacterium Vibrio alginolyticus (T. Nakamura, R. Yuda, T. Unemoto, and E. P. Bakker, J. Bacteriol. 180:3491-3494, 1998). The Ktr transport system consists of KtrB, an integral membrane subunit, and KtrA, a subunit peripherally bound to the cytoplasmic membrane. The genome sequence of Bacillus subtilis contains two genes for each of these subunits: yuaA (ktrA) and ykqB (ktrC) encode homologues to the V. alginolyticus KtrA protein, and yubG (ktrB) and ykrM (ktrD) encode homologues to the V. alginolyticus KtrB protein. We constructed gene disruption mutations in each of the four B. subtilis ktr genes and used this isogenic set of mutants for K ؉ uptake experiments. Preliminary K ؉ transport assays revealed that the KtrAB system has a moderate affinity with a K m value of approximately 1 mM for K ؉ , while KtrCD has a low affinity with a K m value of approximately 10 mM for this ion. A strain defective in both KtrAB and KtrCD exhibited only a residual K ؉ uptake activity, demonstrating that KtrAB and KtrCD systems are the major K ؉ transporters of B. subtilis. Northern blot analyses revealed that ktrA and ktrB are cotranscribed as an operon, whereas ktrC and ktrD, which occupy different locations on the B. subtilis chromosome, are expressed as single transcriptional units. The amount of K ؉ in the environment or the salinity of the growth medium did not influence the amounts of the various ktr transcripts. A strain with a defect in KtrAB is unable to cope with a sudden osmotic upshock, and it exhibits a growth defect at elevated osmolalities which is particularly pronounced when KtrCD is also defective. In the ktrAB strain, the osmotically mediated growth defect was associated with a rapid loss of K ؉ ions from the cells. Under these conditions, the cells stopped synthesizing proteins but the transcription of the osmotically induced proHJ, opuA, and gsiB genes was not impaired, demonstrating that a high cytoplasmic K ؉ concentration is not essential for the transcriptional activation of these genes at high osmolarity. Taken together, our data suggest that K ؉ uptake via KtrAB and KtrCD is an important facet in the cellular defense of B. subtilis against both suddenly imposed and prolonged osmotic stress.

Structurally diverse natural products that cause potassium leakage trigger multicellularity in Bacillus subtilis

“…The phylogenetic conservation of surfactin production and KinC suggests an evolutionary connection between the two. Interestingly, ktrC encodes a putative potassium channel regulator (30 we observed an increase in biofilm formation in MSgg that could still be suppressed by increasing concentrations of potassium (see Fig. S3B).…”

“…One possibility is that the intracellular concentration of potassium decreases, at least temporarily, enough to be sensed. But of course the cell has multiple potassium-uptake mechanisms that should quickly restore intracellular potassium (28)(29)(30). It should be noted that treatment with neither surfactin nor nystatin at the concentration we used (20 and 60 M, respectively) leads to any noticeable decrease in growth rate-an indication that intracellular potassium homeostasis is not severely compromised.…”

Systematic identification of conserved bacterial c-di-AMP receptor proteins

“…This protein has high similarity to the B. subtilis proteins KtrA BS (51% identity) and KtrC BS (63% identity) that, together with their respective membrane components KtrB and KtrD, form potassium transporters (Fig. 1B) (22). SAUSA300_0988 is the only KtrA/C-type protein in S. aureus and was renamed KtrA SA .…”

The structure of the KtrAB potassium transporter

“…The membrane proteins in both superfamilies share the same architecture 2,5-9 : four subunits or repeats, each with the TM-P loop-TM (where TM indicates transmembrane helix) structural motif first seen in the KcsA potassium channel structure 10 , assemble to form an ion pore with a 'selectivity filter' along its central axis. In addition, the cytosolic regulatory proteins of the Trk and Ktr transporters and of the MthK 11,12 and large-conductance Ca 2+ -gated (BK) K + channels [13][14][15] are RCK (regulate conductance of K+) domains.Ktr ion transporters are crucial K + transport systems in some bacteria 16,17 , with a role in resistance to osmotic stress and high salinity by mediating the early uptake of K + (refs 16, 18). The Ktr ion transporters are composed of two essential components 3,16 : a membrane protein (KtrB or KtrD) and a cytosolic regulatory protein (KtrA or KtrC).…”

“…Ktr ion transporters are crucial K + transport systems in some bacteria 16,17 , with a role in resistance to osmotic stress and high salinity by mediating the early uptake of K + (refs 16, 18). The Ktr ion transporters are composed of two essential components 3,16 : a membrane protein (KtrB or KtrD) and a cytosolic regulatory protein (KtrA or KtrC).…”