Regulation of potassium uptake in .

Potassium (K) is an essential physiological element determining membrane potential, intracellular pH, osmotic/turgor pressure, and protein synthesis in cells. Here, we describe the regulation of potassium uptake systems in the oligotrophic α-proteobacterium known as a model for asymmetric cell division. We show that can grow in concentrations from the micromolar to the millimolar range by mainly 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 gene is essential while inactivation does not impact the growth. In contrast, becomes critical but not essential and dispensable for growth in K-limited environments. However, in the absence of , mutations in were selected to improve growth in K-depleted conditions, likely by increasing the affinity of Kup for K. In addition, mutations in the KdpDE two-component system, which regulates expression, suggest that the inner membrane sensor regulatory component KdpD mainly works as a phosphatase to limit the growth when cells reach late exponential phase. Our data therefore suggest that KdpE is phosphorylated by another non-cognate histidine kinase. On top of this, we determined the KdpE-dependent and independent K transcriptome. 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. Here, we show that the oligotroph can support growth at micromolar concentrations of K by mainly using two K uptake systems, the low-affinity Kup and the high-affinity Kdp. Using genome-wide approaches, we also determined the entire set of genes required for 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 , the inner membrane sensor regulatory component KdpD seems to work rather as a phosphatase on the phosphorylated response regulator KdpE~P.