Page 125 - The Biochemistry of Inorganic Polyphosphates

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Gene activity control, development and stress response 109
starvation is thought to be due to the binding of ppGpp to RNA polymerase (Chatterji
et al., 1998). The ppGpp directly inhibits rRNA promoter in vitro (Barker et al., 2001). In
addition to the role of inhibition of ribosome synthesis, ppGpp participates in coordina-
tion of DNA replication and cell division (Schreiber et al., 1995). In ppGpp-deficient relA
spoT mutants, the expression of rpoS is strongly reduced (Lange et al., 1995). PolyP and
ppGpp are factors (Ishihama, 2000) coordinating in the activation of rpoS. A recent review
(Venturi, 2003) analyses the main studies on rpoS transcriptional regulation in E. coli and
Pseudomonas.
However, in some cases these compounds act independent of, or contrary to, rpoS. In
E. coli and S. typhimurium, the regulatory protein leuO, which is potentially involved in
the regulation of many genes, is expressed when bacteria are in the process of transition
from the exponential to the stationary growth phase. LeuO expression is very sensitive to
the cellular level of ppGpp but not dependent on the rpoS (Fang et al., 2000).
The second example is the biosynthesis of antibiotics. The ppGpp is a positive effector
in the synthesis of antibiotics in Streptomyces. The disruption of the ppGpp synthetase relA
gene of Streptomyces coelicolor (Chakraburty and Bibb, 1997) and Streptomyces antibioti-
cus (Hoyt and Jones, 1999) gives phenotypes unable to produce antibiotics. The disruptants
were unable to accumulate ppGpp to the level sufficient for initiation of morphological
differentiation and antibiotics production.
The antibiotic producer Streptomyces lividans possesses a ppk gene, which was cloned
(Chouayekh and Virolle, 2002). Its transcription was only detectable during the late stages
of growth in a liquid minimal medium. A mutant strain interrupted for ppk was character-
ized by increased production of the antibiotic actinorhodin. This production was completely
abolished by the addition of KH 2 PO 4 to the medium. In the ppk mutant strain, this increased
production correlated with enhanced transcription of actII-ORF4 encoding a specific activa-
tor of the actinorhodin pathway. In that strain, the transcription of redD and cdaR, encoding
specific activators of the undecylprodigiosin and calcium-dependent antibiotic biosynthetic
pathways, respectively, was also increased, but to a lesser extent. The enhanced expression
of these regulators did not seem to relate to increased relA-dependent ppGpp synthesis,
as no obvious increase in relA expression was observed in the ppk mutant strain. These
results suggested that the negative regulatory effect exerted by ppk on antibiotic biosyn-
thesis was most probably caused by repression exerted by the endogenous P i , resulting
from the hydrolysis of PolyP synthesized by polyphosphate kinase, on the expression of
specific activators of the antibiotic biosynthetic pathways (Chouayekh and Virolle, 2002).
Earlier, the interaction of PolyP metabolism and antibiotic biosynthesis has been studied in
Streptomyces aureofaciens (Hostalek et al., 1976; Kulaev et al., 1976) and in Streptomyces
levorini (Zuzina et al., 1981) and the competition between PolyP accumulation and antibi-
otic biosynthesis was revealed (Figures 7.5 and 7.6). The low-productive strains contained
a 10-fold higher PolyP level than the high-productive ones. The excess of P i in the culture
medium increased PolyP accumulation and decreased the synthesis of antibiotics (Zuzina
et al., 1981).
There are other examples of the influence of PolyPs on the expression of some genes
omitting rpoS. If the level of cellular PolyP in E. coli was reduced to a barely detectable con-
centrationbyoverproductionofexopolyphosphatase(Shibaetal.,1997),thecellsweremore
sensitive to UV or mitomycin C than the control cells. PolyP accumulation was observed
after treatment with mitomycin C, whereas the PolyP level was below the detectable level