Page 123 - The Biochemistry of Inorganic Polyphosphates

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Regulation of enzyme activities 107
by a long-chain PolyP (Maier et al., 1999). At a concentration of 0.1 % or higher, PolyP
had a bacteriocidal effect on logarithmic-phase cells. This activity was strictly dependent
on active growth and cell division, since PolyP failed to induce lysis in cells treated with
chloramphenicol and in stationary-phase cells, which were, however, bacteriostatically in-
hibited by PolyP. The 0.1 % PolyP inhibited spore germination and outgrowth, and a higher
concentration (1.0 %) was even sporocidal. Addition of Mg 2+ and Ca 2+ could almost com-
pletely block and reverse the antimicrobial activity of PolyP. While DNA replication and
chromosome segregation were undisturbed, electron microscopy revealed a complete lack
of septum formation. It was proposed that PolyP might have an effect on the ubiquitous
bacterial cell division protein FtsZ, whose GTPase activity is known to be strictly dependent
on divalent metal ions. (Maier et al., 1999). The bacteriostatic effect of PolyP on Staphy-
lococcus aureus was also observed (Jen and Shelef, 1986). The addition of PolyP did not
significantly inhibit the growth of Listeria monocytogenes and S. aureus in milk, probably
because of high concentrations of divalent metal cations in this growth medium (Rajkowski
et al., 1994).
Some other effects of PolyP on the important proteins were found, the mechanisms of
which are still unclear. PolyP had a stimulatory effect on the regeneration of GTP-bound
from the GDP-bound form of human and yeast ras proteins. These authors suggested
possible mechanisms of participation of such effects in the regulation of ras-dependent
pathways (De Vendittis et al., 1986).
PolyPs, as well as nucleoside di-, tri- and tetraphosphates and phosphorylated sugars,
caused a dose-dependent (1–5 mM range) delay in the appearance of the cytopathogenic
effect of Clostridium difficile toxin B on human lung fibroblasts. With a longer phosphate
chain, the delay was more pronounced. By analogy with the P site on diphtheria toxin, it
was postulated that C. difficile toxin B contains a PolyP-binding site. This site is separate
from the receptor-binding site but is involved in the interaction of toxin B with cell surfaces
(Florin and Thelestam, 1984).
The effects of PolyPs on the enzymes of RNA metabolism may be a way of participa-
tion of such biopolymers in gene-activity modulation. RNA polymerase isolated from the
stationary-phase cells of E. coli was found to be closely associated with PolyP (Kusano
and Ishihama, 1997). The inhibitory effects of PolyPs on transcription were examined by
using two forms of the holoenzyme, one containing σ 70 (the major sigma-factor for tran-
scription of the genes expressed during exponential cell growth) and the other containing
σ 38 (the sigma-factor operating in the stationary phase). At low salt concentrations, PolyPs
38
inhibited the transcription by both forms of the RNA polymerase, with σ 70 and with σ .At
38
70
high-salt concentrations, the σ -containing enzyme is activated, while the σ -containing
enzyme is unable to function. These results show that PolyPs may play a certain role in the
promoter-selectivity control of RNA polymerase in E. coli growing under high osmolarity
and during the stationary-growth phase.
The polyphosphate kinase was found to be an additional component of E. coli degrado-
some (Blum et al., 1997). This multi-enzyme complex, whose function is RNA processing
and degradation, consists of four major proteins, i.e. endoribonuclease Rnase E, exoribonu-
clease PNPase, RNA helicase and enolase. The ppk-deleted mutant showed an increased
stability of the ompA mRNA. Purified polyphosphate kinase was shown to bind RNA, while
RNA binding was prevented by ATP (Blum et al., 1997). PolyPs were found to inhibit RNA
degradation by the degradosome in vitro. This inhibition was overcome by ADP, required