Page 151 - The Biochemistry of Inorganic Polyphosphates

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Aerobacter aerogenes (Klebsiella aerogenes) 135
phosphotransferase activities declined upon the addition of P i , while both the PolyP-
synthesis activities and PolyP levels rose. Indeed, under most conditions the Acinetobacter
polyphosphate kinase works poorly in reverse. If polyphosphate kinase does not work (or
works poorly) in reverse in vivo, then the cell would be forced to use exopolyphosphatase
and the phosphate–inorganic (Pit) system to recover energy from the PolyP (Kortstee et al.,
2000). It was suggested that the formation of PolyP-producing enzymes is linked to the for-
mation of PolyP-degrading enzymes. Thus, the same conditions (here, P i starvation) which
trigger the induction of exopolyphosphatase lead to the induction of polyphosphate kinase.
When the conditions change (e.g. P i is added), polyphosphate kinase is ready and available
to form PolyP. This type of regulation could also occur with other nutritional stresses, such
as carbon starvation to surplus and anaerobic to aerobic shifts, which occur in EBPR sys-
tems. When high-phosphate-grown cells of the strictly aerobic A. johnsonii were incubated
anaerobically, their PolyP content was degraded and P i was excreted. The bacterium have
two enzymes catalysing PolyP degradation, i.e. polyphosphate:AMP phosphotransferase
and exopolyphosphatase (Kortstee et al., 2000). In A. johnsonii, PolyP serves as an en-
ergy source during anaerobiosis by (i) direct synthesis of ATP via the polyphosphate:AMP
phosphotransferase/adenylate kinase pathway, and (ii) generation of a proton motive force
+
by the coupled excretion of MeHPO 4 and H . Exopolyphosphatase may enhance the latter
energy recycling mechanism by providing the efflux process with a continuous supply of
P i and divalent metal cations (Kortstee et al., 2000).

8.4 Aerobacter aerogenes (Klebsiella aerogenes)

Aerobacter aerogenes (Klebsiella aerogenes) does not accumulate PolyPs in a sufficient
amount under normal growth conditions but sometimes begins to accumulate PolyPs under
unfavourable growth conditions (Smith et al., 1954). A detailed investigation has been
carried out into the effects of different conditions and growth phases on the PolyP content
in this organism (Wilkinson and Duguid, 1960). Quite a substantial accumulation of PolyP
occurred at pH 4.5 with sulfur deficiency, after P i had been added to the phosphorus-starved
culture. For PolyP accumulation by this bacterium, in addition to the P i and energy source,
the presence of K and Mg 2+ ions in the culture medium was essential. The interrelation
+
between PolyP and nucleic acid metabolism has also been observed (Wilkinson and Duguid,
1960). The total amount of PolyP in the cells increased when the growth and nucleic acid
synthesis ceased, but the accumulated PolyP (in the form of volutin granules) disappeared
after the growth had been resumed.
The fundamental work on PolyP metabolism in K. aerogenes has been carried out by
Harold and co-workers (Harold, 1963a,b, 1964, 1966; Harold and Harold, 1963, 1965;
Harold and Sylvan, 1963). It was shown that the effect of growth conditions on PolyP
metabolism is mediated by two entirely different mechanisms.
The first mechanism was realized when the growth was ceased by nutrient deprivation
or stress conditions. For example, Figure 8.9(a) shows the gradual PolyP accumulation
during sulfur deprivation, while the growth and nucleic acid biosynthesis are suppressed
(Harold, 1966). A similar observation was made (Harold, 1963b) during the examination
of auxotrophic mutants, which required uracil or methionine for normal growth. PolyP was
accumulated in the culture medium in the absence of these components, while on addition