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Bacteria have evolved sophisticated Pi transport systems which combine high affinity with coupling to metabolic energy. This review discusses the current evidence concerning the physiological, biochemical, and molecular properties of these Pi transport systems in prokaryotes. Major developments of the past years will be presented with emphasis on three kinds of issues. First, work on Pi transport in Escherichia coli and the polyphosphate-accumulating Acinetobacter johnsonii has assigned a novel biochemical mechanism and provided additional descriptive information for the transport of Pi and divalent cations. It is therefore appropriate to summarize these new facts and emphasize their general relevance for pro- and eukaryotic cells. Second, recent work on the bioenergetics of Pi transport in A. johnsonii has demonstrated the profound role of the transmembrane Pi gradient in energy transducing processes such as the accumulation of solutes, and the generation of a proton motive force. These findings and their significance for the survival of the cell during metabolic stress conditions will be discussed. Finally, polyphosphate-accumulating microorganisms play a valuable role in biotechnological applications, such as in wastewater treatment. As such organisms are still underrepresented in current molecular microbiological studies, the investigations in A. johnsonii described here may serve as a useful precedent for those to come. |
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