Regulation of potassium starvation.

<p>(A) The tracking approach to detect potential regulators of homeostasis. Parameters which are constant in the minimal model are now considered as input functions. A parameter is called a potential regulator if it can be chosen to recover (“track”) the experimental time courses. (B,C) The predicted activity changes for Pma1 (B) and the bicarbonate reaction system (C) in response to potassium starvation. (D) Time course of ATPase activity for Pma1. (E) Time course of gene expression for the <i>NCE103</i> gene encoding carbonic anhydrase in the wild type strain. Confirmatory qRT-PCR measurements yield a fold increase of the mRNA level in the wild type after 60 minutes of potassium starvation. For comparison, the expression in non-starved <i>trk1,2</i> double mutant with respect to the wild type strain is depicted. The mRNA levels for <i>NCE103</i> in <i>trk1,2</i> double mutants growing at are higher by a factor of compared to the wild type strain (qRT-PCR measurements).</p

Relationship of external and internal potassium.

<p>(A) Cells grown in 50 mM KCl were resuspended in 0.1, 0.2 and 0.5 mM KCl and the time course of internal potassium was monitored. The lines show the data fit obtained from the reverse tracking algorithm. (B) Internal potassium concentration in cells grown overnight at different external potassium concentrations. The steady state concentrations from (A) are indicated as squares.</p

PMA1 mutants with decreased expression and ATPase activity.

<p>Strains RS514 (wild type, WT), RS515 (<i>pma1–204</i>) and RS516 (<i>pma1–205</i>) were grown in YNB-based medium (supplemented with adenine and histidine) with 2% galactose to maintain Pma1 activity from plasmid pYCp50-GALp::PMA1. Cells were diluted to an OD600 of 0.04 in Translucent -free medium (plus with 2% glucose) containing 1 mM or 50 mM KCl. Growth was monitored for 17 h. Data represent the growth ratio at 1 and 50 mM KCl and are mean SEM from 3 determinations.</p

Optical densities during starvation.

<p>Optical densities for the wild type and the <i>trk1,2</i> double mutant corresponding to the potassium starvation experiments of <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1002548#pcbi-1002548-g001" target="_blank">Figure 1A</a>.</p

Proposed mechanism of potassium homeostasis.

<p>Changes of the external potassium concentration are sensed by an unidentified sensor system either directly or indirectly, e.g, via the membrane potential, internal potassium or pH changes. The sensor signal triggers a modulation of proton fluxes using the bicarbonate reaction system and the Pma1 proton pump as regulators.</p