Page 224 - The Biochemistry of Inorganic Polyphosphates

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Polyphosphates in chemical and biological evolution
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These functions are predominant in animal cells, where PolyP participates mainly in the
transport across the membranes and in the regulation of gene expression.
Therefore, future comparative investigations of PolyP metabolism in archae, bacteria and
eukaryotes will provide better understanding of the evolution of the functions of PolyPs.
In microbial cells, inorganic PolyP plays a significant role increasing cell resistance to
unfavourable environmental conditions and regulating different biochemical processes;
whereas in animal cells, which possess a hymoral and neuorous regulatory mechanisms,
the PolyP functions become narrow but do not disappear. The ability to synthesize high-
molecular-weight PolyPs is apparently of great importance even in the higher animals and
plants, since their cells are thereby rendered less dependent on external factors.
The PolyP metabolism in eukaryotic cells has specific peculiarities in each cellular com-
partment. For example, a large amount of evidence has been obtained for yeast, suggesting
thatthesynthesisanddegradationofPolyPineachspecializedorganelleandcompartmentof
the cells is mediated by different sets of enzymes. This is consistent with the endosymbiotic
hypothesis of eukaryotic cell origin.
According to this hypothesis (Margulis, 1993), the eukaryotic cell is a result of sym-
biosis of different prokaryotic cells, where mitochondria originated from eubacteria, and
chloroplasts – from cyanobacteria, and vacuoles – from archae.
The main argument in favour of this hypothesis is the presence in chloroplasts and
mitochondria of DNA, which is different from the nuclear DNA and similar to the DNA
of prokaryotes, as well as the similarity of chloroplast, mitochondrial and bacterial ri-
bosomes and their significant difference from cytoplasmic ribosomes of eukaryotic cells.
The chloroplasts and mitochondria were found to be close to bacterial cells in additional
other biochemical features: the presence in their membranes of phospholipid cardiolipin,
which is absent in the plasma membrane of eukaryotes, and ATPases of one and the same
type F 1 F 0 .
The homology of V-ATPases and pyrophosphatases of the vacuoles and plasma mem-
branes of archae indicates a possibility of endosymbiotic descent of vacuoles from ancient
representatives of this domain (Nelson, 1992).
PolyP metabolism in mitochondria and chloroplasts has been little studied as yet. The
question of retention in these organelles of some peculiarities of PolyP metabolism char-
acteristic of bacterial cells is still open. In particular, computer analysis of the genomes
of chloroplasts and mitochondria in some plants and yeast has not revealed any sequences
similar to the genes of bacterial polyphosphate kinase ppk1 and exopolyphosphatase ppx
(Kulakovsky,unpublishedresults).Thesesequencesarehighlyconservedinbacteria(Reizer
et al., 1993; Kornberg et al., 1999), and their absence points to a possible loss of the corre-
sponding genes in the course of evolution.
It should be mentioned that the ribosomes of chloroplasts and mitochondria synthesize
only a comparatively small part of the proteins required for the formation and function of
these organelles. This may be due to the fact that the process of symbiosis occurred in a very
distant epoch and many genes have shifted from autonomous genomes into nuclei since
that time.
For example, the genes encoding putative guanosine 3 ,5 -bispyrophosphate (ppGpp)

synthase–degradase, which is a member of the RelA-SpoT family of bacterial proteins,
were identified in the nuclear genomes of the unicellular photosynthetic eucaryote Chlamy-
domonas reinhardtii (Kasai et al., 2002) in the halophyte Suaeda japonica (Yamada et al.,