Potassium transport and its regulation in escherichia coli: studies on transcriptional regulation of the kdp operon

Abstract

Potassium (K+) is the predominant intracellular cation in all life forms including bacteria newlinesuch as Escherichia coli. In E. coli, the cytoplasmic K+ concentration under normal growth newlineconditions is maintained at around 150 mM. There js evidence that several metabolic activities newlinelike enzyme function, DNA protein interactions and pH homeostasis occur optimally at this newlinecytoplasmic concentration of K+. In addition, increased cytoplasmic accumulation of K+ in newlinebacteria is one of the primary means of adaptation to environments of high external osmolarity, newlineand intracellular K+ concentration ([K+]i) is believed to be an important determinant of cell newlineturgor. Bacteria can regulate [K+]i to cope with fluctuations in medium osmolarity. newlineTo achieve accumulation or dissipation of K+, E. coli possesses a number ofK+ transport newlinesystems. There are three systems involved in K+ efflux namely the KefB, KefC and a turgor newlineactivated K+ efflux system. The activities of the KefB and KefC systems aid in regulation of newlinecytoplasmic pH while the turgor activated K+ efflux system is involved in release of cytoplasmic newlineK+ during adaptation to a low osmolarity environment. newlineTrkA, TrkD (also called Kup) and Kdp are the three K+ uptake systems of E. coli. The newlinefirst two systems are constitutively expressed and are encoded by unlinked genes on the E. coli newlinechromosome. These are classified as low-affinity K+ uptake systems. The activities of the TrkA newlineand TrkD systems serve to satisfy the K+ needs of the cell over a wide range of external K+ newlineconcentrations ([K+]e). newlineKdp is an inducible high-affinity multisubunit K+ uptake system. Kdp has a Km forK+ of newlinearound 2 J..LM and its synthesis is induced at the level of transcription when [K+]e becomes newlinelimiting for growth. The four subunits of the Kdp complex are· the products of the kdpF ABC newlineoperon (kdp operon). A strain with mutations in the TrkA, TrkD and the Kdp systems (trkA trkD newlinekdp - triple K+ transporter mutant) exhibits very low-level residual K+ uptake activity attributed