Regulation of potassium homeostasis in Caulobacter crescentus
Potassium (K+) is an essential physiological element determining membrane potential, intracellular pH, osmotic/turgor pressure, and protein synthesis in cells. Nevertheless, K+ homeostasis remains poorly studied in bacteria. Here we describe the regulation of potassium uptake systems in the oligotrophic -proteobacterium Caulobacter crescentus known as a model for asymmetric cell division. We show that C. crescentus can grow in concentrations from the micromolar to the millimolar range by essentially using two K+ transporters to maintain potassium homeostasis, the low affinity Kup and the high affinity Kdp uptake systems. When K+ is not limiting, we found that the kup gene is essential while kdp inactivation does not impact the growth. In contrast, kdp becomes critical but not essential and kup dispensable for growth in K+-limited environments. However, in the absence of kdp, mutations in kup were selected to improve growth in K+-depleted conditions, likely by improving the affinity of Kup for K+. In addition, mutations in the KdpDE two-component system, which regulates kdpABC expression, suggest that the inner membrane sensor regulatory component KdpD works as a kinase in early stages of growth and as a phosphatase to regulate transition into stationary phase. Our data also show that KdpE is not only phosphorylated by KdpD but also by another non-cognate histidine kinase. On top of this, we determined the KdpE-dependent and independent K+ transcriptome and identified the direct targets of KdpE. Together, our work illustrates how an oligotrophic bacterium responds to fluctuation in K+ availability. ImportancePotassium (K+) is a key metal ion involved in many essential cellular processes. Its transport and regulation have been mainly studied in the bacterial model species Escherichia coli and Bacillus subtilis. Here we show that the oligotroph Caulobacter crescentus can support growth at lower K+ concentrations by mainly using two K+ uptake systems, the low-affinity Kup and the high-affinity Kdp. Interestingly, in the absence of Kdp, point mutations in Kup was selected to increase affinity for K+, which improved growth in K+-depleted conditions. Using genome-wide approaches, we also determined the entire set of genes required for C crescentus to survive at low K+ concentration as well as the full K+-dependent regulon. Finally, we found that the transcriptional regulation mediated by the KdpDE two-component system is unconventional since unlike E. coli, the inner membrane sensor regulatory component KdpD works rather as a phosphatase on the phosphorylated response regulator KdpE[~]P. To our knowledge, this is the first comprehensive study of K+ homeostasis in bacteria.