Page 131 - The Biochemistry of Inorganic Polyphosphates

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Gene activity control, development and stress response 115
Evidence for the participation of (p)ppGpp and PolyP-dependent systems in the regula-
tion of development of prokaryotes with a complicate life cycle, Myxococcus coralloides
(Gonzales et al., 1989) and Myxococcus xanthus (Singler and Kaiser, 1995; Harris et al.,
1998), has also been obtained.
Generation of high levels of (p)ppGpp in response to amino acid starvation in E. coli re-
sults in a significant accumulation of PolyP (Kuroda et al., 1997). This accumulation can be
attributed to the inhibition by pppGpp and/or ppGpp of PolyP hydrolysis by exopolyphos-
phatase. PolyP accumulation under stress required high levels of ppGpp, independent of
whether they are generated by RelA (active during stress response) or SpoT (expressed dur-
ing P i starvation) (Rao et al., 1998). Accumulation of PolyP requires the functional PHOB
gene and higher levels of (p)ppGpp (Rao et al., 1998; Ault-Riche et al., 1998). In E. coli,
the genes ppk and ppx are in the same operon, which results in a coordinated regulation of
their activities (Rao et al., 1998).
Various mechanisms, providing the participation of PolyP in gene expression regulation
processes, have been proposed (Figure 7.8). First, polyphosphate kinase may be involved
in regulation of the level of nucleoside triphosphates and deoxynucleoside triphosphates,
while this enzyme can convert GDP and other nucleoside diphosphates to nucleotide triphos-
phates using PolyP. Secondly, this enzyme may influence mRNA stability, regulating RNA
degradation in degradosomes. Thirdly, PolyP is directly involved in the regulation of RNA
polymerase expression and activity. Finally, the (p)ppGpp and PolyP metabolism and rpoS
expression are closely interrelated. In addition, there are some genes which are regulated
by ppGpp or PolyP, independent of the rpoS network.
It should be noted that different bacteria might have different predominant mechanisms
of PolyP participation in survival under stress and in the stationary-growth phase, or other
mechanisms that have not been studied yet. For instance, in Helocobacter pilory the pppGpp
level does not rise as a result of amino acid starvation (Scoarughi et al., 1999). Com-
pletely sequenced genomes of several obligately parasitic organisms (Treponema pallidum,
Chlamydia species and Rickettsia prowazekii), as well as the known archaea genomes,
do not contain rel-like genes, and the role of ppGpp in these organisms may probably be
diminished (Mittenhuber, 2001).
These recent data provide good evidence for the essential role of PolyPs in regula-
tion of biochemical processes and the overcoming of stress and starvation by prokaryotic
cells.

7.7.2 In Lower Eukaryotes

For eukaryotic microorganisms, the involvement of PolyPs in biochemical regulation under
stress has also been observed. For example, the involvement of vacuolar PolyP in survival
under osmotic or alkaline stress has been shown in algae and fungi. In the alga Dunaliella
salina, alkalinization of the cytoplasm results in a massive hydrolysis of PolyP, resulting in
pH stat. Various authors have suggested that the hydrolysis of PolyP provides the pH-stat
mechanism to counterbalance the alkaline stress (Bental et al., 1990; Pick et al., 1990; Pick
and Weis, 1991).
The role of PolyP as a buffer was demonstrated in N. crassa under osmotic stress where
the hypoosmic shock produced a rapid hydrolysis of the PolyP with an increase in the
concentration of cytoplasmic phosphate (Yang et al., 1993).