Page 119 - The Biochemistry of Inorganic Polyphosphates

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Cell-envelope formation and function 103
requires divalent cations, and only certain cations are fit for this – Mg ,Ca ,Mn 2+ and
2+
2+
Sr . All of these cations form strong ionic bonds with phosphate and can ‘cross-bridge’ the
2+
phosphate residues of DNA and PolyP. For DNA uptake to occur, cells must return to normal
growth media. Examination of the thermotrophic fluorescence spectrum of the cells therein
has revealed a rapid decrease in the intensity of the 56 C fluorescence peak, indicating that
◦
the PolyP–PHB complexes are being removed from the membrane. Hence, a mechanism
of DNA transmembrane transfer has been proposed. As PolyP is retrieved by cytoplasmic
enzymes, it may draw the bound DNA molecule into and through the PHB channel. From
this viewpoint, various procedures for competence development and DNA transformation
are simply resourceful methods to change the direction of PolyP movement within the PHB
pore from outward to inward. The cells are first placed into an environment that leads to
a substantial increase in PolyP–PHB, with a sufficient number of divalent cations to bind
DNA to PolyP. Then, they are transferred to a medium where they ordinarily sustain much
lower levels of membrane complexes, thus inducing an inward flow of PolyP. In support of
this hypothesis, a single-stranded donor DNA was found in complex with PHB, when DNA
uptake in E. coli RR1 was interrupted in the first few minutes (Reusch et al., 1986; Huang
and Reusch, 1995; Reusch, 1999a).
Little is known of the ways of biosynthesis and insertion in the membranes of PolyP–
PHB–Ca 2+ complexes. In polyphosphate kinase 1 mutants of E. coli, the amounts of the
complexes did not change (Castuma et al., 1995). Therefore, the PolyP in the complexes is
synthesized not by polyphosphate kinase 1 but by another enzyme. E. coli strain, which lacks
the AcrA component of a major multi-drug resistance pump, had greatly reduced amounts
of the complexes and was defective in its ability to maintain sub-micromolar levels of
free Ca 2+ in the cytoplasm (Jones et al., 2003). This indicates that the AcrAB transporter
may have a novel, hitherto undetected, physiological role, either directly in the membrane
assembly of the PHB complexes or the transport of a component of the membrane, which
is essential for assembly of the complexes into the membrane.
It should be noted that complexes of different proteins with PolyPs (Schr¨oder et al.,
1999) or PHBs (Reusch et al., 2002) were found in cells. The prokaryotic histone-like
protein, E. coli H-NS, and eukaryotic calf thimus histone proteins, Hq, H2A, H2B, H3
and H4, were found to be post-translationally modified by conjugation with short-chain
PHBs. The presence of these compounds in proteins with similar functions in such diverse
organismssuggeststhatPHBsplayacertainroleinshapingthestructureand/orinfacilitating
the function of these important proteins (Reusch et al., 2002). It cannot be excluded that
complexes of proteins with PolyP, PHB, and both polymers together, may be found in
different cell compartments, not only in the membranes, and have any regulatory role,
which needs further investigation.

7.5 Cell-Envelope Formation and Function

7.5.1 Polyphosphates in the Cell Envelopes of Prokaryotes

Thecellenvelopesofbacteriaplayanessentialroleinbacterialvirulence,surfaceattachment
and biofilm formation (O’Toole et al., 2000). This cell compartment possesses PolyPs, and
thereby its role in the above functions was intensively investigated. The conclusion was