The extremely halophilic archaeon Halobacterium salinarum R1 responds to potassium limitation by expression of the...

Halobacterium species balance high external osmolality by the accumulation of almost equimolar amounts of KCl. Thus, steady K(+) supply is a vital prerequisite for life of these extreme halophiles. So far, K(+) is reported to enter the halobacterial cell only passively by use of potential-driven uniporters. However, the genome of both the extreme halophilic archaeon Halobacterium sp. NRC-1 and H. salinarum R1 comprises one single gene cluster containing the genes kdpFABC coding for homologs of the bacterial ATP-driven K(+) uptake system KdpFABC, together with an additional ORF so far annotated as cat3 in Halobacterium sp. NRC-1 and as UspA protein in H. salinarum R1 (the ORF is only referred to as cat3 in the following). Deletion of the kdpFABCcat3 genes led to a reduced ability to grow under limiting K(+) concentrations, whereas real-time RT-PCR measurements revealed cat3-dependent high expression rates of the Kdp system in case of external K(+) depletion. Synthesis of the KdpFABC complex enables H. salinarum R1 to grow under extreme potassium-limiting conditions of >20 microM K(+). These results provide the first experimental evidence of an ATP-driven K(+) uptake system in Halobacterium. Moreover, H. salinarum R1 was shown to further adapt to K(+) limitation by a significant decrease of the intracellular K(+) level, which suggests a rather complex mechanism of K(+) homeostasis, in which the adaptation of cellular K(+) concentrations and the concomitant transcriptional regulation of genes coding for a high-affinity ATP-driven K(+) uptake system ensure the essential potassium supply under limiting conditions.