The Kdp-ATPase system and its regulation

K+, the dominant intracellular cation, is required for various physiological processes like turgor homeostasis, pH regulation etc. Bacterial cells have evolved many diverse K+ transporters to maintain the desired concentration of internal K+. In E. coli, the KdpATPase (comprising of the KdpFABC complex), encoded by the kdpFABC operon, is an inducible high-affinity K+ transporter that is synthesised under conditions of severe K+ limitation or osmotic upshift. The E. coli Up expression is transcriptionally regulated by the KdpD and KdpE proteins, which together constitute a typical bacterial two-component signal transduction system. The Up system is widely dispersed among the different classes of bacteria including the cyanobacteria. The ordering of the kdpA, kdpB and kdpC is relatively fixed but the kdpD/E genes show different arrangements in distantly related bacteria. Our studies have shown that the cyanobacterium Anabaena sp. strain L-31 possesses two Up operons, kdp1 and kdp2, of which, the later is expressed under K+ deficiency and desiccation. Among the regulatory genes, the kdpD ORF of Anabaena L-31 is truncated when compared to the kdpD of other bacteria, while a kdpE-like gene is absent. The extremely radio-resistant bacterium, Deinococcus radiodurans strain R1, also shows the presence of a naturally short kdpD ORF similar to Anabaena in its Up operon. The review elaborates the expression of bacterial Up operons in response to various environmental stress conditions, with special emphasis on Anabaena. The possible mechanism(s) of regulation of the unique kdp operons from Anabaena and Deinococcus are also discussed.