Page 133 - The Biochemistry of Inorganic Polyphosphates

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Gene activity control, development and stress response 117
It was suggested that the accumulation of amines within vacuoles (in response to amine-
induced alkaline stress) activates a specific exopolyphosphatase which hydrolyses long-
chain PolyPs to PolyP 3 (Pick and Weis, 1991). This enzyme appears to be activated at
neutral or mild alkaline pH levels and repressed at the physiological intravacuolar acidic
pH level. To date, two enzymes, which could catalyse this process, have been purified.
An exopolyphosphatase, which hydrolyses long-chain PolyPs but not PolyP 3 , and has an
optimal pH at 7–7.5, was purified from yeast vacuoles (Andreeva et al., 1998b). An en-
dopolyphosphatase (Kumble and Kornberg, 1996) could also be involved in this process.
One of the possible ways for the involvement of PolyPs in stress overcoming and bio-
chemical regulation is their interactions with the second messengers. In eukaryotes, sec-
ond messengers such as phosphoinositides (Mitchell et al., 1996; Wera et al., 2001) and
diadenosinetetra-, penta- and hexapolyphosphates (Kisselev et al., 1998) are probably in-
terrelated with PolyPs by analogy with the (p)ppGpp and PolyP interactions in bacteria.
Diadenosine tetraphosphate was found to accumulate in yeast cells under stress caused
by exposure to cadmium or heat shock (Baltzinger et al., 1986, Rubio-Texeira et al., 2002).
The diadenosine hexa- and pentaphosphates and hydrolases have an additional function
in S. cerevisiae, namely, the efficient hydrolysis of diphosphorylated inositol polyphos-
phates (Safrany et al., 1999). Thus, the above second messengers may effectively interact
with one another. In addition, their metabolism may be related to PolyP metabolism in
some cases. The yeast exopolyphosphatase PPX1 is capable of hydrolysing adenosine–5 -

tetraphosphate and guanosine–5 -tetraphosphate (Kulakovskaya et al., 1997; Guranowsky
4
1

et al., 1998), while diadenosine–5 ,5 -P ,P -tetraphosphate α,α-phosphorylase (Booth and

Guidotti, 1995) may bind the metabolism of the above compounds and PolyP. In addition,
the chloroplasts of the eukaryotic alga Chlamydomonas reinhardtii was found to possess a
gene encoding a putative guanosine–3 ,5 -bispyrophosphate (ppGpp) synthase–hydroladase

(Kasai et al., 2002). This gene exhibited a marked similarity to eubacterial members of the
RelA-SpoT family of proteins and the authors suggested that eubacterial stringent control
mediated by ppGpp has been conserved during evolution of the chloroplast from a photo-
synthetic bacterial symbiont (Kasai et al., 2002). It is probable that some of the regulatory
mechanisms in which PolyPs are involved may be similar in eubacteria and chloroplasts or
the mitochondria of eukaryotes.
The participation of PolyPs in development processes and regulation of gene activity
is, probably, one of the most important functions of these compounds in eukaryotic mi-
croorganisms. The mechanism of this involvement is as yet still little studied; however,
many facts confirm this concept. The first data which provide evidence for the involvement
of PolyPs in switching on and off large groups of genes were obtained for fungi. It was
shown that during the process of sporulation in the fruiting bodies of the fungus Agaricus
bisporus very large amounts of relatively low-molecular-weight PolyPs accumulate at the
actual site of the basidia (Kritsky et al., 1965a,b). Significant changes in nuclear PolyPs
were observed during this process. Degradation of high-molecular-weight PolyPs to low-
molecular fragments in the nuclei during sporulation was observed in the fungi Agaricus
bisporus, Neurospora crassa (Kritsky and Kulaev, 1963, Kritsky and Belozerskaya, 1968;
Kritsky et al., 1965a,b, 1970, 1972) and Physarum polycephalum (Pilatus et al., 1989). The
relationship between PolyPs and nucleic acids metabolism in the cells of lower eukaryotes
was discovered many years ago, although the precise mechanisms of this relationship is still
obscure. The utilizing of PolyP primarily for the biosynthesis of RNA was demonstrated