Page 109 - The Biochemistry of Inorganic Polyphosphates

15:39
Char Count= 0
March 9, 2004
WU095/Kulaev
WU095-07
Phosphate reserve 93
by genetic regulation and increase in the dosage of E. coli genes encoding polyphosphate
kinase 1, acetate kinase, and phosphate-inducible transport systems (PSTS, PSTC, PSTA,
and PSTB) and by genetic inactivation of ppx encoding exopolyphosphatase. The best
recombinant strains of E. coli eliminated approximately two- and threefold more P i from
the medium than the control strain (Hardoyo et al., 1994). These strains accumulated in
the cells approximately 10-fold more P i than the control strain. The phosphorus content
of these recombinant strains reached a maximum of 16 % of dry biomass. About 65 % of
cellular phosphorus was stored as PolyP (Ohtake et al., 1994). These data suggest that the
systems providing PolyP accumulation in bacteria include many genes in addition to those
encoding the major bacterial PolyP metabolizing enzymes, i.e. polyphosphate kinase and
exopolyphosphatase.
In some culture conditions, extracellular PolyP was identified as a good source of phos-
phate (Curless et al., 1996). Using a typical medium in a high-cell-density fermentation of
E. coli, 40 % higher cell density was obtained when using PolyP instead of P i as a phos-
phate source (Curless et al., 1996). It is probable that the expression of specific porins
allows PolyP transfer from the culture medium into the cells. The outer membrane porin
PhoE of E. coli (Bauer et al., 1989) and the OprO porin of Pseudomonas aeruginosa (Siehnel
et al., 1992; Hancock et al., 1992), induced by phosphate starvation, are examples of proteins
which prefere PP i and PolyP rather than P i .

7.1.2 In Eukaryotes

The accumulation of phosphate reserves as PolyPs and their use at phosphate starvation
also occur in eukaryotic microorganisms. The yeast Saccharomyces cerevisiae (Liss and
Langen, 1962; Kulaev and Vagabov, 1983) and Neurospora crassa (Kulaev and Afanasieva,
1969, 1970) are characterized by the phenomenon of phosphate overplus. These accumulate
higher contents of PolyPs after phosphate starvation, followed by transfer to a phosphate-
containing medium. Such processes touch upon all different PolyP fractions of eukaryotic
microbial cells (Kulaev and Afanasieva, 1969, 1970; Kulaev and Vagabov, 1983; Vagabov
et al., 2000).
The increase in PolyP level in yeast may be due to phosphate uptake stimulation. Cells of
Candida humicola demonstrated a 4.5-fold increase in phosphate uptake from the medium
and accumulated 10-fold more PolyP during growth at pH 5.5, when compared with growth
at pH 7.5 (McGrath and Quinn, 2000). Further details on PolyP accumulation and utilization
in eukaryotes are given in Chapter 8.
Whereas mainly cytosolic PolyP performs the function of phosphorus reservation in
bacteria, in eukaryotic microorganisms phosphorus is also reserved as PolyP in other cell
compartments. Under yeast growth on a medium without phosphate, the PolyP content
drops by more than an order in the cytosol, vacuoles and cell walls (Kulaev and Vagabov,
1983; Kulaev et al., 1999). PolyP granules of the cytosol quickly disappear after the yeast
has been placed in a phosphate-deficient medium. In a P i -deficient medium, a sharp decrease
of the PolyP level, both in whole cells and in vacuoles, was noted, and after 7 h of starvation
the PolyP level in vacuoles decreased by 85 %, which indicates an active utilization of the
entire PolyP pool for the needs of the cell under these growth conditions (Kulaev et al.,
1999; Trilisenko et al., 2002).