Absence of ppGpp Leads to Increased Mobilization of Intermediately Accumulated Poly(3-Hydroxybutyrate) in Ralstonia eutropha H16
Authors: Juengert Janina R. , Borisova Marina , Mayer Christoph , Wolz Christiane , Brigham Christopher J. , Sinskey Anthony J. , Jendrossek Dieter 
Published: April 2017

In this study, we constructed a set of
Ralstonia eutropha
H16 strains with single, double, or triple deletions of the (p)ppGpp synthase/hydrolase (
spoT1
), (p)ppGpp synthase (
spoT2
), and/or polyhydroxybutyrate (PHB) depolymerase (
phaZa1
or
phaZa3
) gene, and we determined the impact on the levels of (p)ppGpp and on accumulated PHB. Mutants with deletions of both the
spoT1
and
spoT2
genes were unable to synthesize detectable amounts of (p)ppGpp and accumulated only minor amounts of PHB, due to PhaZa1-mediated depolymerization of PHB. In contrast, unusually high levels of PHB were found in strains in which the (p)ppGpp concentration was increased by the overexpression of (p)ppGpp synthase (SpoT2) and the absence of (p)ppGpp hydrolase. Determination of (p)ppGpp levels in wild-type
R. eutropha
under different growth conditions and induction of the stringent response by amino acid analogs showed that the concentrations of (p)ppGpp during the growth phase determine the amount of PHB remaining in later growth phases by influencing the efficiency of the PHB mobilization system in stationary growth. The data reported for a previously constructed Δ
spoT2
strain (C. J. Brigham, D. R. Speth, C. Rha, and A. J. Sinskey, Appl Environ Microbiol 78:8033–8044, 2012,
https://doi.org/10.1128/AEM.01693-12
) were identified as due to an experimental error in strain construction, and our results are in contrast to the previous indication that the
spoT2
gene product is essential for PHB accumulation in
R. eutropha
.

IMPORTANCE
Polyhydroxybutyrate (PHB) is an important intracellular carbon and energy storage compound in many prokaryotes and helps cells survive periods of starvation and other stress conditions. Research activities in several laboratories over the past 3 decades have shown that both PHB synthase and PHB depolymerase are constitutively expressed in most PHB-accumulating bacteria, such as
Ralstonia eutropha
. This implies that PHB synthase and depolymerase activities must be well regulated in order to avoid a futile cycle of simultaneous PHB synthesis and PHB degradation (mobilization). Previous reports suggested that the stringent response in
Rhizobium etli
and
R. eutropha
is involved in the regulation of PHB metabolism. However, the levels of (p)ppGpp and the influence of those levels on PHB accumulation and PHB mobilization have not yet been determined for any PHB-accumulating species. In this study, we optimized a (p)ppGpp extraction procedure and a high-performance liquid chromatography–mass spectrometry (HPLC-MS)-based detection method for the quantification of (p)ppGpp in
R. eutropha
. This enabled us to study the relationship between the concentrations of (p)ppGpp and the accumulated levels of PHB in the wild type and in several constructed mutant strains. We show that overproduction of the alarmone (p)ppGpp correlated with reduced growth and massive overproduction of PHB. In contrast, in the absence of (p)ppGpp, mobilization of PHB was dramatically enhanced.