Page 129 - The Biochemistry of Inorganic Polyphosphates

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March 9, 2004
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WU095/Kulaev
WU095-07
Gene activity control, development and stress response 113
The ppk1 null mutation affects the adherence properties of bacteria. The Salmonella ppk
mutant showed a 20–35 % decrease of adherence to abiotic surface (polystyrene) relative to
the wild type. The mutant was only half as invasive in epithelium cells (Kim et al., 2002).
The P. aeruginosa ppk1 mutant was moderately defective at an early stage of attachment to
polystyrene surface, but the biofilm maturation was greatly affected (Rashid et al., 2000b).
The ppk mutant of Porphyromonas gingivalis, which seems to be important in the etiology of
periodontitis, was attenuated in biofilm formation on poly(vinyl chloride) and glass, while
the insertion of an intact ppk gene copy restored its biofilm formation (Chen et al., 2002).
In view of the fact that the motility and biofilm formation of pathogens are essential
to invade and establish systemic infections in host cells, these data suggest a crucial and
essential role of polyphosphate kinase or PolyP in bacterial pathogenesis.
The quorum-sensing mechanism allows bacteria to coordinate the expression of partic-
ular genes. In Pseudomonas aeruginosa, a complex quorum-sensing circuitry, associated
with RpoS expression, is required for cell-density-dependent production of many secreted
virulence factors, including LasB elastase. The overexpression of relA activated the expres-
sion of rpoS in P. aeruginosa and led to premature, cell-density-independent LasB elastase
production. It was suggested that the stringent response can activate two quorum-sensing
systems of P. aeruginosa independent of cell density (Van Delden et al., 2001).
In as much as the ppk1 mutant of P. aeruginosa is defective in three types of motility,
surface attachment and biofilm differentiation, Rashid et al. (2000b) determined the levels
of the quorum-sensing molecules, AI-1 (N-3-(oxododecanoyl)-L-homoserine lactone) and
AI-2 (N-butyryl-L-homoserine lactone). Their levels in theppk1 mutant were reduced to
∼ 50 % of those of the parent type. The complementation of the mutant with the ppk1 gene
doubled the parent strain level (Rashid et al., 2000b). The extracellular virulence factors,
elastase and rhamnolipid, also decreased in the ppk1 mutant and were restored when the
mutant was complemented by ppk-containing plasmid (Rashid et al., 2000b). As a result,
the lethality of the P. aeruginosa ppk mutant in the burn-mouse model decreased to ∼ 7%
of the lethality of the parent strain (Rashid et al., 2000b).
Thus, PolyPs and polyphosphate kinase affect the development processes in many
bacteria.
One example of the adaptation of bacteria to an unfavourable environment is their re-
sponse to amino acid starvation. In an environment rich in amino acids, cells do not produce
enzymes of amino acid synthesis. However, in the case of a nutritional downshift in the en-
vironment, cells must use their own proteins as sources of amino acids for building enzymes
required for amino acid biosynthesis pathways (Gottesman and Maurizi, 2001).
The mutant of E. coli lacking ppk1 exhibited an extended lag phase of growth when
shifted from a rich to minimal medium. Supplementation of amino acids to the minimal
medium abolished the extended growth lag of the mutant. Levels of the stringent response
factor, ppGpp, increased in response to the nutritional downshift, but unlike in the wild
type, the levels were sustained in the mutant. These results suggested that the mutant
was impaired in the induction of amino acid biosynthetic enzymes. The rate of protein
degradation increased in response to the nutritional downshift in the wild type, whereas
it did not in the mutant. Thus, polyphosphate kinase ppk1 is required to stimulate protein
degradation and for adaptation to amino acid starvation in E. coli (Kuroda et al., 1999).
Convincing evidence was obtained that protein degradation in E. coli during amino acid
starvation depends on the ATP-dependent proteases Lon and Clp (Kuroda et al., 2001).
Mutations in Lon and Clp proteases produced the same phenotype as ppk1 mutation – the