ISSN 0003-6838, Applied Biochemistry and Microbiology, 2018, Vol. 54, No. 3, pp. 288–293. © Pleiades Publishing, Inc., 2018.
Improved Tolerance of Escherichia coli to Propionic Acid
by Overexpression of Sigma Factor RpoS
and P. Tian
Beijing Key Laboratory of Bioprocess, College of Life Science and Technology, Beijing University of Chemical Technology,
Beijing, 100029 People’s Republic of China
Received September 11, 2017
Abstract⎯Propionic acid (PA) is an economically important compound, but large-scale microbial produc-
tion of PA confronts obstacle such as acid stress on microbial cells. Here, we show that overexpressing sigma
factor RpoS improves the acid tolerance of Escherichia coli. Four genes including rpoS, fur, pgi and dnaK
(encoding RNA polymerase sigma factor, ferric uptake regulator, phosphoglucoisomerase, and chaperone,
respectively) were independently overexpressed in E. coli. The recombinant E. coli overexpressing rpoS
showed the highest PA tolerance. This strain could grow in M9 medium at pH 4.62, whereas wild type E. coli
survived only at pHs above 5.12. Moreover, in the shake-flask cultivation, the E. coli strain overexpressing
rpoS grew faster than wild type. Notably, the minimum inhibitory concentration of PA for this recombinant
strain was 7.81 mg/mL, which was 2-fold higher in comparison with wild type. Overall these results indicated
that overexpression of sigma factor rpoS significantly enhanced E. coli tolerance to PA.
Keywords: Escherichia coli, propionic acid, rpoS, pgi, tolerance
Propionic acid (PA) is an economically valuable
carboxylic acid with wide applications. PA not only is
food preservative, it also serves as the substrate for
manufacturing cellulose plastics. Currently, industrial
production of PA mainly relies on petrochemical route
[1, 2]. Due to depletion of petroleum and deteriora-
tion of environment, there is an increased interest in
fermentation production of PA from renewable
resources. Among diverse PA-synthesizing species,
propionibacteria, especially those from colon, have
been exploited for bioproduction of PA. Despite ardu-
ous endeavors, current PA titer, and therefore fermen-
tation yield and productivity, remain too low for com-
mercialization. The reasons behind are multifaceted.
One is the low microbial resistance to metabolites
such as PA, which severely halts cell growth. To miti-
gate metabolites stress on microbial cells, a series of
measures have been taken, including optimization of
fermentation process [1, 3], cell immobilization [4, 5],
and in situ product recovery . Despite these efforts,
PA titer and productivity are still below expectation.
Engineering microbial tolerance to metabolites
represents a challenge in the development of efficient
bioconversion system. Previous studies have shown
that a series of regulators participate in acid tolerance.
For instance, the RNA polymerase sigma factor RpoS
confers acid tolerance response induced by cell growth
at low pHs or by cells entering to stationary phase [7, 8].
RpoS also participates in heat and salt tolerance as
well as the survival of Escherichia coli in dry environ-
ments . It is clear that RpoS helps E. coli to cope
with different stresses. In addition to RpoS, ferric
uptake regulator Fur also contributes to acid toler-
ance , as the fur mutant exhibits a significant
decrease in viability when cells are exposed to high
concentration of acids. Furthermore, scientists also
found that overexpression of the dihydroxyacetone
kinase DhaK in Lactococcus lactis improved cell
growth, heat stress tolerance and lactic acid fermenta-
tion efficiency . Stephanopoulos group in Massa-
chusetts Institute of Technology (USA) proposed a
global transcription machinery engineering strategy
based on RNA polymerase sigma factor, and the
engineered yeast showed increased tolerance to etha-
nol . All above mentioned regulators can globally
disturb cellular metabolism.
In view of above information, we hypothesized that
microbial acid tolerance could be ameliorated by
reprogramming cellular metabolism. In this study,
4 genes were independently overexpressed in E. coli.
The aim of the study was to analyze the cell growth,
protein expression and minimum inhibitory concen-
tration (MIC) of recombinants to examine microbial
tolerance to PA. Shake-flask cultivation of recombi-
The article is published in the original.