Page 147 - The Biochemistry of Inorganic Polyphosphates

Char Count= 0
20:32
March 9, 2004
WU095-08
WU095/Kulaev
Pseudomonas aeruginosa 131
It should be noted that the exopolyphosphatase gene of E. coli, i.e. ppx, was found
‘downstream’ of the gene for polyphosphate kinase ppk1 (Akiyama et al., 1993). Tran-
scription of the ppx gene depended on the ppk1 promoters, indicating a single operon of
ppk1 and ppx (Akiyama et al., 1993). This proposes a coordinate regulation of these en-
zymes. The fact that one of the promoters has a homology with the Pho box indicated that
PolyP metabolism might be regulated by P i . The effects of some mutations indicated above
support this suggestion.
The effects of mutations and overexpression of ppk1 and ppx were studied by Keasling
and co-workers (Keasling et al., 1998, 2000; Sharfstein et al., 1996; Van Dien et al., 1997;
Van Dien and Keasling, 1998). If the PolyP operon of E. coli was overexpressed on a high-
copy plasmid under the control of its native promoter, the ppk activity increased during
phosphate starvation and dropped after the P i shift, while ppx activity was the highest when
P i was in surplus. Thus, in such a transformant, the Pho regulon was probably not involved
in the expression of ppk and ppx, while utilization of PolyP during starvation and its partial
replenishment after the P i shift was retained (Sharfstein et al., 1996; Van Dien et al., 1997;
Keasling et al., 1998; Van Dien and Keasling, 1998). Thus, the interaction of the genes
directly involved in PolyP metabolism and the Pho regulon is very complicated and needs
further investigation. A mathematical model was proposed to study the multiple aspects of
the phosphate-starvation response of E. coli (Van Dien and Keasling, 1998).

8.1.4 The Effects of Nutrition Deficiency and
Environmental Stress

The influence of nutrition limitation and stress conditions on the PolyP content in E. coli
was studied by Kornberg and co-workers and described in detail in a number of reviews
(Kornberg, 1995, 1999; Rao and Kornberg, 1999; Kornberg et al., 1999), and earlier in
Chapter 7. Here, we will point out only the most important facts. First, E. coli, subjected
to nutritional or osmotic stress in a rich medium or to nitrogen exhaustion, had a large
and dynamic accumulation of PolyP (Rao and Kornberg, 1996; Kuroda et al., 1997; Rao
et al., 1998; Ault-Riche et al., 1998). E. coli accumulated large amounts of PolyPs in media
deficient in both P i and amino acids. For example, Figure 8.5 shows PolyP accumulation in
the presence of serine hydroxamate inducing amino acid starvation (Kuroda et al., 1997).
This accumulation is explained by the high level of ppGpp under these conditions and the
inhibition of PolyP hydrolysis by this compound (Kuroda et al., 1997). The accumulation
of PolyPs under stresse seems to be a reaction for overcoming the unfavourable growth
conditions.
Based on the available data, a tentative model (Figure 8.6) of the mechanisms responsible
for PolyP accumulation in E. coli has been proposed (Ault-Riche et al., 1998; Rao and
Kornberg, 1999).

8.2 Pseudomonas aeruginosa

In Pseudomonas aeruginosa, a specific regulation of PolyP accumulation by the regulatory
protein AlgR2 was revealed. This protein positively regulated the production of alginate,