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The nature of polyphosphate
Inorganic polyphosphate (polyP) is a linear, unbranched polymer of orthophosphate residues linked by phosphoanhydride bonds (Figure 1.1). PolyP ranges in size from three to over one thousand orthophosphate residues (Kornberg et al., 1999). PolyP is widespread in bacteria and yeasts and has been found in plant and animal cells (polyP is also formed by dehydration and condensation of phosphate at the elevated temperatures of benthic and volcanic vents (Kornberg et al., 1999).
PolyP was first found in yeast cells by Liebermann (1888). Further work by Wiame (1947),
Kornberg (1956) and others through the 1940s and 1950s established the role of polyP, or 'volutin' as it was then known in the accumulation of phosphate and in energy storage by microorganisms. PolyP was observed in many microorganisms as metachromatic particles and was historically used as a diagnostic tool for certain pathogens such as Corynebacterium diphtheriae (Robinson & Wood, 1986). PolyP, like other anions, shifts the absorption of basic dyes such as toluidine blue, to a shorter wavelength (630 to 530nm) therefore giving rise to a metachromatic effect. When viewed by electron microscopy intracellular polyP appears as dark, electron dense granules. The presence of polyP in cells may also be detected by other techniques such as 31P-NMR analysis (Glonek et al., 1971) and by fluorescence of 4-6-diamidino-2-phenylindole (DAPI) (Allan & Miller, 1980).
Under certain conditions, such as nutrient limitation, during stationary phase or osmotic stress some microorganisms have been shown to accumulate relatively large amounts of polyP (Tzeng & Kornberg, 1998). In Acinetobacter johnsonii polyP may account for up to 30% of the dry biomass (Deinema et al., 1985). Such levels of polyP are well in excess of the normal metabolic requirements for phosphate and indicate an important role of polyP in response to changes in nutritional status or environmental conditions (Kornberg et al., 1999).