The constitutive K + pump in Serratia marcescens

Abstract

Transport of K + and H + in the anaeronically and aerobically grown bacterium Serratia marcescens has been studied. The volumes of one cell of the anaerobically and aerobically grown bacterium were 3.7 · 10 −13 cm 3 and 2.4 · 10 −13 cm 3, respectively. Irrespective of the growth conditions the bacteria manifested the same respiration rate. However, the values of membrane potential for the anaerobically and aerobically grown bacterium were different and equal to −130 mV and −175 mV (interior negative), respectively, in the absence of an exogenic energy source. KCN + DCCD decreases Δψ down to almost zero in both species. DCCD alone decreases Δψ partially in anaerobes and increases Δψ in aerobes, whereas KCN alone reduces Δψ partially in both species. The introduction of glucose into the medium containing K + reduces the absolute value of Δψ to |−160| mV in aerobes and to |−20| mV in anaerobes. The effect is not observed without external K +. In the presence of arsenate a Δψ is not reduced after the addition of glucose. At pH 7.5–7.8 the ATP level in aerobes grows notably faster than in anaerobes. The H + extrusion becomes intensified when K + uptake is activated by the increase in external osmotic pressure. Apparent K m and V max for K + accumulation are 1.2 mM and 0.4 mM · min −1 · g −1. The decrease of Δψ by glucose or KCN + DCCD have no effect on the K + uptake whereas CCCP inhibits potassium accumulation. At the same time, arsenate stabilizes the Δψ value, but blocks K + uptake. The accumulation of K + correlates with the potassium equilibrium potential of −200 mV calculated according to the Nernst equation, whereas the Δψ measured was not more than |−25| mV. The calculated H + ATP stoichiometry was 3.3 for aerobes. It was assumed that a constitutive K + pump having a K + ATP ratio equal to 2 or 3 operates in S. marcescens membranes.