Energetic consequences of two mutations in Escherichia coli K+ uptake systems for growth under potassium-limited conditions in the chemostat

The energetics of growth of two Escherichia coli strains (TK 2240 and TK 2242) differing in Km of the high-affinity potassium uptake system and lacking the low-affinity system were studied in the chemostat under potassium-limited conditions. The results were compared with the results obtained previously (Mulder, M.M., Teixeira de Mattos, M.J., Postma, P.W. and Van Dam, K. (1986) Biochim. Biophys. Acta 851, 223-228) with the wild-type FRAG-1, having two potassium uptake systems, and FRAG-5, a mutant which lacks the high-affinity potassium uptake system. We postulated that the high-affinity potassium uptake system was able to generate such a steep gradient across the membrane that the low-affinity system would act in reverse, thus creating a futile cycle of potassium ions at the cost of energy. As a result, FRAG-1 would show a higher ATP turnover at all growth rates tested than the mutant FRAG-5, in which strain the proposed futile cycle is interrupted because of the lack of the high-affinity system. It is shown here that the results obtained with TK 2240 and TK 2242 are in line with our hypothesis of futile potassium cycling. Under our experimental conditions, the yield on potassium was not dependent on the kinetic parameters of the uptake systems. The (thermodynamic) energy demand of the uptake systems determined the carbon substrate conversion required to achieve this yield.