B

Aa

La

b

a

A

R

A

A

A

S

K

A

A

B

E

I

AtwtmaH

h1B

b r a z i l i a n j o u r n a l o f m i c r o b i o l o g y 4 8 (2 0 1 7) 809–814

ht tp : / /www.bjmicrobio l .com.br /

iotechnology and Industrial Microbiology

novel expression vector for the secretion ofbaecin in Bacillus subtilis

i Lia,∗,1, Lan Mua,1, Xiaojuan Wanga,1, Jingfeng Yua,1, Ruiping Hua,∗, Zhen Lib

College of Basic Medicine, Inner Mongolia Medical University, Hohhot, Inner Mongolia, ChinaCollege of Horticulture, China Agricultural University, Haidian District, Beijing, China

r t i c l e i n f o

rticle history:

eceived 13 September 2016

ccepted 31 January 2017

vailable online 3 June 2017

ssociate Editor: Gisele Monteiro de

ouza

eywords:

baecin

ntimicrobial peptides

acillus subtilis

xpression

a b s t r a c t

This study aimed to describe a Bacillus subtilis expression system based on genetically mod-

ified B. subtilis. Abaecin, an antimicrobial peptide obtained from Apis mellifera, can enhance

the effect of pore-forming peptides from other species on the inhibition of bacterial growth.

For the exogenous expression, the abaecin gene was fused with a tobacco etch virus pro-

tease cleavage site, a promoter Pglv, and a mature beta-glucanase signal peptide. Also, a B.

subtilis expression system was constructed. The recombinant abaecin gene was expressed

and purified as a recombinant protein in the culture supernatant. The purified abaecin did

not inhibit the growth of Escherichia coli strain K88. Cecropin A and hymenoptaecin exhib-

ited potent bactericidal activities at concentrations of 1 and 1.5 �M. Combinatorial assays

revealed that cecropin A and hymenoptaecin had sublethal concentrations of 0.3 and 0.5 �M.

This potentiating functional interaction represents a promising therapeutic strategy. It pro-

vides an opportunity to address the rising threat of multidrug-resistant pathogens that are

recalcitrant to conventional antibiotics.

© 2017 Sociedade Brasileira de Microbiologia. Published by Elsevier Editora Ltda. This is

an open access article under the CC BY-NC-ND license (http://creativecommons.org/

licenses/by-nc-nd/4.0/).

ntroduction

ntimicrobial peptides (AMPs) are important components ofhe innate immune defense against microbial pathogens in aide range of organisms.1 Abaecin is a major AMP found in

he honeybees. It was originally isolated from the insect Apis

ellifera.2 The bumblebee long-chain proline-rich peptide

baecin can reduce the minimal inhibitory concentrations.3

owever, the expression of abaecin is extremely limited in

∗ Corresponding authors.E-mails: 609532398@qq.com (L. Li), 783674348@qq.com (R. Hu).

1 These authors contributed equally to this study.ttp://dx.doi.org/10.1016/j.bjm.2017.01.009517-8382/© 2017 Sociedade Brasileira de Microbiologia. Published by EY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/)

honeybees. Therefore, a highly efficient expression system isneeded for abaecin to allow its commercial application.

Abaecin consists of a single polypeptide chain of commonamino acids and is well suited for economical productionthrough the application of recombinant DNA technology orpeptide synthesis.4 It can potentiate the activity of pore-forming AMPs from other species, for example, cecropin A

and stomoxyn.5 This potentiating functional interaction rep-resents a promising therapeutic strategy because the potencyand range of combinations of AMPs are likely to extend well

lsevier Editora Ltda. This is an open access article under the CC.

i c r o

810 b r a z i l i a n j o u r n a l o f m

beyond the capabilities of individual peptides.6 It provides anopportunity to address the rising threat of multidrug-resistantpathogens that are recalcitrant to conventional antibiotics.7

The expression of foreign genes in Escherichia coli oftenleads to the formation of densely packed denatured pep-tide molecules in the form of insoluble particles calledinclusion bodies.8 Inclusion body proteins are devoid ofbiological activity. An elaborate process involving solubi-lization, refolding, and purification is needed to partiallyrecover a functionally active product.9 A yeast expressionsystem, such as Pichia pastoris, may solve the problem ofpost-translational modifications.10 It needs significant invest-ment during the later period of batch cultivation, rendering ituneconomical.11

The Bacillus subtilis expression system does not have thedisadvantages of the two aforementioned expression sys-tems and can potentially serve as an efficient expressionhost,12 especially for the secretion of heterologous proteins.13

Moreover, the secreted foreign proteins usually remain inbiologically active forms,14 and the downstream purifica-tion is greatly simplified.15 Also, it has other advantages,such as theoretically higher yield, no aggregation of theproduct, and the possibility for continuous cultivation andproduction.16

This study aimed to describe a B. subtilis expression sys-tem based on B. subtilis cells genetically modified with a smalltobacco etch virus (TEV) protease and two-cistron expressionvector gene of abaecin fused to TEV.17

Materials and methods

Enzymes, chemicals, and antibiotics

T4 DNA ligase, Taq DNA polymerase, and all restrictionenzymes were purchased from Promega (WI, USA). The pro-tein marker was obtained from TaKaRa Biotechnology (Shiga,Japan). Antibiotics were purchased from Sigma (MO, USA). Allthe other chemicals used were of the highest grade commer-cially available.

Bacterial strains and plasmids

The gene encoding the antibacterial peptide abaecin, withthe previously reported sequence FVPYNPPRPGQSKPFPSF-PGHGPFNPK IQWPYPLPNPGH,18 was synthesized using a TEVprotease cleavage locus gene, a beta-glucanase mature pep-tide, and signal peptide gene (gmp and gsp) as a full-lengthnucleotide by standard solid-phase methods at the AUGCTBiotechnology Company (Beijing, China). The recombinantplasmid named ABA containing the abaecin gene, a TEVprotease cleavage locus gene, and a beta-glucanase maturepeptide (gmp) with the signal peptide (gsp) were supplied bythe AUGCT Biotechnology Company (Beijing, China). The E. coliDH5�, shuttle vector pGJ103, and promoter Pglv were obtained

from National Feed Engineering Technology Research Cen-ter (Beijing, China). Preparation of plasmid DNA from E. colicells and transformation of B. subtilis were carried out usingstandard procedures.19

b i o l o g y 4 8 (2 0 1 7) 809–814

Plasmid construction and expression

After cloning using T vector, an intact plasmid includingthe promoter Pglv was inserted into plasmid pGJ103 (3.2 kb),resulting in the construction of pGP (3.45 kb). The plasmidcontaining the genes of abaecin and TEV protein, with gmpand gsp, was digested with EcoRI and BamHI, while pGP wasdigested with EcoRI and BamHI. The two digested productswere linked by T4 DNA ligase, yielding recombinant plasmidpGPA (Fig. 1). The operon, including the inducible promoterspac, and the TEV protease gene were inserted between thesite BamHI and SacI of plasmid pGPA. The plasmid pGPA wastransformed into E. coli DH5�, and the positive transformantwas screened at a final concentration of 5 �g/mL chloromycin.The positive clones were further identified with the restrictionendonuclease digestion of BamHI and SacI. The sequence wasidentified by the SinoGenoMax Company (Beijing, China).

The plasmid pGPA extracted from the positive transfor-mant was transformed into the expression vector B. subtilis1A747 with chloramphenicol resistance. The clones werepicked with shaking at 37 ◦C overnight in Luria-Bertani (LB)broth containing 50 �g/mL of chloramphenicol. After this, theculture was inoculated into new LB broth at a ratio of 1:100with shaking at 37 ◦C until the optical density at 600 nmreached 0.8 − 1.0. The culture was induced with 1% glucoseand incubated at 37 ◦C for an additional 24 h with a rota-tion speed of 250 rpm. Then, the supernatant was collectedby centrifugation. Various pH values of the culture medium(pH 3.0 − 7.0) were tested for the optimal expression of recom-binant proteins. It was found that pH 6.0 of the medium wasthe optimal condition for the expression of abaecin (data notshown).

Purification of the recombinant abaecin

The supernatant was first filtered twice using an Amiconultrafiltration device (Millipore, MA, USA). The supernatant,containing proteins ranging from 3 to 10 kDa, was dia-lyzed overnight in 0.1 M sodium acetate and then appliedto a CM-Sepharose CL-6B column (Pharmacia Biosciences,NJ, USA) pre-equilibrated with 0.1 M sodium acetate (pH5.0). The column was washed with 0.1 M acetate buffer, andthe proteins were eluted with a linear gradient of 0.1–1.0 Msodium acetate (pH 5.0). A sample (100 �L) of the filtratewas collected and applied to semi-preparative reversed-phasehigh-performance liquid chromatography (HPLC) on a C18column (250 × 4.6 mm2, 5 �m, and 300 × A), equilibrated in0.1% trifluoroacetic acid and 18% acetonitrile. The bound pro-tein was eluted with a linear gradient of acetonitrile (18%– 45%, v/v) in 0.1% trifluoroacetic acid. The flow rate was1.0 mL/min, and the absorbance of eluted protein was mon-itored at 280 nm. The peak of the chimeric abaecin elutedat 31% acetonitrile from reversed-phase HPLC was deter-mined by analyzing fractions on Tricine-sodium dodecyl

20

sulfate–polyacrylamide gel electrophoresis (SDS-PAGE). Thefractions containing abaecin were collected for the next step.The sequence of recombinant abaecin was determined by theSinoGenoMax Company (Beijing, China)

b r a z i l i a n j o u r n a l o f m i c r o b i o l o g y 4 8 (2 0 1 7) 809–814 811

Apal

EcoRI TEV BamHI

BamHI

Spac TEV protease

Sac lApal

Sphl Cm

BamHI

SacI

SphI

ApaI

Cm

CoIEI

Pglv

gsp

gmp Abaecin gene

Spac

TEV protease

B.sub rep

CoIEI

Pglv

ABA

gsp gmp Abaecin gene

ApalXholSa/ IEcoRVEcoRIPst lSmalBamHISpelXbalNotlSac/ ISacI

EcoRIPst lSmalBamHIspelXbalNot lSac/ lSac l

EcoRI

Pglv

Pglv

EcoRI

Abaecingene

Sphl Cm

CoIEI

ORI–

ORI–

ORI+

ORI+

B.sub rep

B.sub rep

pGJ103

PGP

PGPA

Pglv

Fig. 1 – Construction of recombinant plasmid pGPA. After cloning using T vector, an intact plasmid including the promoterPglv was inserted into plasmid pGJ103 (3.2 kb), resulting in the construction of pGP (3.45 kb). The plasmid containing thegenes of abaecin and TEV protein, with gmp and gsp, was digested with EcoRI and BamHI, while pGP was digested withEcoRI and BamHI. The two digested products were linked by T4 DNA ligase, yielding recombinant plasmid pGPA.

A

GCT(wn

ntimicrobial activities

rowth inhibition assays: E. coli K88ac was purchased from thehina Veterinary Culture Collection Center (Beijing, China).he bacteria were cultivated and incubated in LB medium

0.5% NaCl, 0.5% yeast extract, 1% tryptone, pH 7). All strainsere then stored at −80 ◦C with 20% sterile glycerol untileeded.21

Growth inhibition was determined in sterilized 96-wellplates in a final volume of 200 �L using the microdilution assayas described previously.22 A stock solution of the peptideswas diluted tenfold in the culture medium and 100 �L of LBmedium. The bacteria (2 × 107 to 4 × 107 cells/mL) were addedto 100 �L of LB medium and peptide solution (serial twofold

dilutions). The OD600 was measured every 20 min for 16 h inan Eon Microplate Spectrophotometer (BioTek Instruments,

i c r o b i o l o g y 4 8 (2 0 1 7) 809–814

100

90

80

70

60

50

40

30

20

10

0999.0 1799.4 2599.8

4308.03

3400.2

Mass (m/z)

Inte

nsity

(%

)

4200.6 5001.0

Fig. 3 – Analysis of purified recombinant abaecin byelectrospray ionization mass spectrometry. Electrosprayionization mass spectrometry of the purified abaecindemonstrated a single, nondispersed signal with a

812 b r a z i l i a n j o u r n a l o f m

VT, USA). Control cultures with no AMPs were included in theassays.

Results

Plasmid constructions and expression

The recombinant plasmid ABA with a restriction enzyme EcoRIcleavage site at the 3′-terminus and a BamHI cleavage site atthe 5′-terminus was linked with pGP (digested with EcoRI andBamHI) by T4 DNA ligase. As shown in Fig. 1, the constitutiveexpression vector pGPA, containing the operon of induciblepromoter spac and TEV protease gene, was constructed. B. sub-tilis supplied a favorable base for expression. After a three-stepconstruction process, the recombinant plasmid containing thefusion gene of abaecin and TEV was transformed into B. subtilis1A747 for expression.

During the initial stage of expression, the expressed fusionprotein including abaecin and TEV had no antimicrobial activ-ity. This ensured that the expressed product would not exert adeleterious effect on B. subtilis host strain, and the host strainwould continuously express the fusion protein within a cer-

tain period. Also, the fusion protein was expressed at a higherlevel when isopropyl �-d-1-thiogalactopyranoside (IPTG) wasadded. The cultivation was continued for a certain period after

M 1 2KD

20.1

14.47.8

5.8

3.3

4.3

3 4

Fig. 2 – Expression of abaecin in Bacillus subtilis 1A747. Theanalysis of purified abaecin on Tricine-SDS-PAGE andWestern Blot revealed its molecular mass to be about 4 kDa.(a) SDS-PAGE analysis of abaecin expressed. Lane 1, totalcell protein (uninduced); lane 2, total cell protein (induced);lane M, low-range molecular mass marker (TaKaRa, Shiga,Japan). (b) Western blot analysis of abaecin expressed. Lane3, total cell protein (uninduced); lane 4, total cell protein(induced). The arrow indicates the position of recombinantabaecin.

molecular mass of 3.9 kDa.

induction with IPTG, and then the culture conditions wereadjusted (data not shown) to release TEV protease into theculture medium. The targeted expressed protein abaecin wasactivated on the hydrolysis of TEV protease to TEV. As shownin Fig. 2, the recombinant abaecin was expressed in B. subtilis ata quite high level. The expressed protein was released directlyinto the culture medium and obtained from the supernatantafter centrifugation.

Purification of the modified abaecin

Following highly efficient purification including ultrafiltrationand reversed-phase HPLC, pure chimeric antibacterial peptideabaecin was obtained from the culture medium. The analysisof purified abaecin on Tricine-SDS-PAGE revealed its molec-ular mass to be about 4 kDa (Fig. 2). Electrospray ionizationmass spectrometry of the purified abaecin demonstrated asingle, nondispersed signal with a molecular mass of 3.9 kDa(Fig. 3). The sequence of recombinant abaecin was identifiedas consistent with the theoretical sequence.

Potency of the AMPs in bacterial growth inhibition assays

E. coli strain K88 showed no susceptibility in the presence ofup to 200 �M abaecin and entered the exponential growthphase∼4 h after the initiation of cultivation. The conclu-sion was consistent with the findings of Rahnamaeian.23 Incontrast, cecropin A and hymenoptaecin exhibited potentbactericidal activities at concentrations of 1 and 1.5 �M,respectively. The sublethal concentrations of the AMPs weredetermined by preparing serial dilutions and repeating thegrowth inhibition assays. This revealed that cecropin Aand hymenoptaecin had sublethal concentrations of 0.3 and0.5 �M, respectively (data not shown). Combinatorial assays

were carried out by supplementing the sublethal doses of eachpore-forming peptide with 20 �M abaecin (Fig. 4).

b r a z i l i a n j o u r n a l o f m i c r o b i

a

b

1.4

1.2

1

0.8

0.6

0.4

0.2

0

1.4

1.2

1

0.8

0.6

0.4

0.2

0

2 4

CeropinA 0.3µMAbaecin 20µM

Hymenotaecin 0.5 µMAbaecin 20µMControlAbaecin+Hymenoptaecin

ControlAbaecin+CeropinA

6 8

Hour(h)

Hour(h)

Abs

orba

nce(

OD

600)

Abs

orba

nce(

OD

600)

10 12 14

2 4 6 8 10 12 14

Fig. 4 – Escherichia coli growth inhibition assays. (a) E. colistrain K88 in the mid-logarithmic phase was incubatedwith the medium (control) or with abaecin andhymenoptaecin, alone or in combination. (b) E. coli strainK88 in the mid-logarithmic phase was incubated with themedium (control) or with abaecin and cecropin A, alone orin combination. The growth rate was determined bym

D

Hgdepeieda

cpsbsutrioawt

r

improved zonula occludens-1 expression. Asian-Australas J

easuring the optical density of the culture at 600 nm.

iscussion

oneybee AMPs are a family of small polypeptides withreat potential for antibacterial application.24 However, toate, a few reports are available detailing the heterologousxpression of recombinant honeybee AMPs. Therefore, in theresent study, B. subtilis was used as a host for the high-levelxpression and secretion of chimeric peptide abaecin. Thentroduction of TEV into the fusion gene offered a favorablenvironment for the expression of abaecin. Abaecin was pro-uced in B. subtilis with the help of a special operon withoutny damage to the host strain.

Producing AMPs (such as honeybee AMPs) with high effi-iency is a significant challenge for developing commercialroducts.25 In the present study, the B. subtilis expressionystem was adopted for the constitutive expression of recom-inant abaecin.25 To facilitate the expression of abaecin, thisystem included the TEV protease gene, which could releasender appropriate conditions and ensure the cleavage of thearget gene.26 Also, the modified expression system did notequire an inducer during expression27 and did not producenclusion bodies.23 Therefore, the system had the advantagesf high efficiency of expression at low cost, easy operation, andpplication to industrial production. The recombinant abaecin

as expressed at a quite high level (up to about 1 g/L) in bac-

erial cell culture.

o l o g y 4 8 (2 0 1 7) 809–814 813

The analysis of purified abaecin using Tricine-SDS-PAGErevealed its molecular mass to be about 4 kDa, which wasconsistent with the theoretical molecular mass of 3.966 kDaobtained from electrospray ionization mass spectrometry.

It has been shown previously that abaecin alone does notaffect bacterial proliferation at concentrations of up to 200 �M,but can potentiate the activity of hymenoptaecin (a pore-forming peptide also from the bumblebee) and thus reducethe minimum inhibitory concentrations of hymenoptaecinrequired for membrane permeabilization.28 This interactioncan be specific, given that the two AMPs are naturally co-expressed in response to a bacterial challenge, but can alsorepresent a more general mechanism.

Antibiotics are currently widely used as therapeutic agentsand growth stimulants for farm animals. However, the useof antibiotics in animal feed should be completely banneddue to concerns regarding the presence of residues in ani-mal products and the development of bacterial resistance toantibiotics.29 Consequently, the development of alternativesto antibiotics needs considerable attention.30 The fact thatabaecin could enhance the antimicrobial activity against spe-cific tested organisms t suggests that one potential applicationof abaecin may be in the livestock industry, since bacteria suchas staphylococci, E. coli, and salmonella are some of the pri-mary organisms causing infectious diseases in livestock.31,32

In summary, a novel B. subtilis expression system forabaecin based on B. subtilis cells genetically modified with aTEV protease and the two-cistron expression vector gene ofabaecin fused to TEV was developed in this study. Abaecincould enhance the effect of pore-forming peptides from otherspecies on the inhibition of bacterial growth. These find-ings indicated that the B. subtilis expression system could beapplied as a powerful tool for producing abaecin at a largerscale, which is expected to become a useful antimicrobial oreven therapeutic agent in animals or even human beings. Afurther study would test this hypothesis through some prac-tical feeding trials.

Conflict of interest

The authors declared that they have no conflict of interest.

e f e r e n c e s

1. Xu J, Zhong F, Zhang Y, et al. Construction of Bacillus subtilisstrain engineered for expression of porcinebeta-defensin-2/cecropin P1 fusion antimicrobial peptidesand its growth-promoting effect and antimicrobial activity.Asian-Australas J Anim Sci. 2016.

2. Casteels P, Ampe C, Riviere L, et al. Isolation andcharacterization of abaecin, a major antibacterial responsepeptide in the honeybee (Apis mellifera). Eur J Biochem.1990;187:381–386.

3. Gu MJ, Song SK, Park SM, Lee IK, Yun CH. Bacillus subtilisprotects porcine intestinal barrier from deoxynivalenol via

Anim Sci. 2014;27:580–586.4. Guan C, Cui W, Cheng J, et al. Construction of a highly active

secretory expression system via an engineered dual

i c r o

814 b r a z i l i a n j o u r n a l o f m

promoter and a highly efficient signal peptide in Bacillussubtilis. N Biotechnol. 2016;33:372–379.

5. Gupta M, Rao KK. Phosphorylation of DegU is essential foractivation of amyE expression in Bacillus subtilis. J Biosci.2014;39:747–752.

6. He Q, Fu AY, Li TJ. Expression and one-step purification ofthe antimicrobial peptide cathelicidin-BF using the inteinsystem in Bacillus subtilis. J Ind Microbiol Biotechnol.2015;42:647–653.

7. Hemila H, Pakkanen R, Heikinheimo R, Palva ET, Palva I.Expression of the Erwinia carotovorapolygalacturonase-encoding gene in Bacillus subtilis: role ofsignal peptide fusions on production of a heterologousprotein. Gene. 1992;116:27–33.

8. Heng C, Chen Z, Du L, Lu F. Expression and secretion of anacid-stable alpha-amylase gene in Bacillus Subtilis by SacBpromoter and signal peptide. Biotechnol Lett.2005;27:1731–1737.

9. Huang X, Li Z, Du C, Wang J, Li S. Improved expression andcharacterization of a multidomain xylanase fromThermoanaerobacterium aotearoense SCUT27 in Bacillus subtilis.J Agric Food Chem. 2015;63:6430–6439.

10. Ji SH, Gururani MA, Chun SC. Expression analysis of ricepathogenesis-related proteins involved in stress responseand endophytic colonization properties of gfp-tagged Bacillussubtilis CB-R05. Appl Biochem Biotechnol. 2014;174:231–241.

11. Jia M, Xu M, He B, Rao Z. Cloning, expression, andcharacterization of l-asparaginase from a newly isolatedBacillus subtilis B11-06. J Agric Food Chem. 2013;61:9428–9434.

12. Joyet P, Derkaoui M, Poncet S, Deutscher J. Control of Bacillussubtilis mtl operon expression by complexphosphorylation-dependent regulation of the transcriptionalactivator MtlR. Mol Microbiol. 2010;76:1279–1294.

13. Khezri M, Jouzani GS, Ahmadzadeh M. Fusarium culmorumaffects expression of biofilm formation key genes in Bacillussubtilis. Braz J Microbiol. 2016;47:47–54.

14. Lee SJ, Pan JG, Park SH, Choi SK. Development of a stationaryphase-specific autoinducible expression system in Bacillussubtilis. J Biotechnol. 2010;149:16–20.

15. Li RK, Chen P, Ng TB, et al. Highly efficient expression andcharacterization of a beta-mannanase from Bacillus subtilis inPichia pastoris. Biotechnol Appl Biochem. 2015;62:64–70.

16. Li Y, Li Z, Yamanaka K, et al. Directed natural productbiosynthesis gene cluster capture and expression in themodel bacterium Bacillus subtilis. Sci Rep. 2015;5:9383.

17. van den Berg S, Lofdahl PA, Hard T, Berglund H. Improvedsolubility of TEV protease by directed evolution. J Biotechnol.2006;121:291–298.

18. Liu Y, Lu F, Chen G, et al. High-level expression, purificationand characterization of a recombinant medium-temperature

alpha-amylase from Bacillus subtilis. Biotechnol Lett.2010;32:119–124.

19. Lu Q, Ma J, Rong H, et al. Cloning, expression, purification,crystallization and preliminary crystallographic analysis of

b i o l o g y 4 8 (2 0 1 7) 809–814

5-aminolaevulinic acid dehydratase from Bacillus subtilis.Acta Crystallogr Sect F Struct Biol Cryst Commun.2010;66:1053–1055.

20. Ma P, Patching SG, Ivanova E, et al. Allantoin transportprotein, PucI, from Bacillus subtilis: evolutionaryrelationships, amplified expression, activity and specificity.Microbiology. 2016;162:823–836.

21. Michna RH, Commichau FM, Todter D, Zschiedrich CP, StulkeJ. SubtiWiki-a database for the model organism Bacillussubtilis that links pathway, interaction and expressioninformation. Nucleic Acids Res. 2014;42:D692–D698.

22. Ogawa T, Iwata T, Kaneko S, Itaya M, Hirota J. An induciblerecA expression Bacillus subtilis genome vector for stablemanipulation of large DNA fragments. BMC Genomics.2015;16:209.

23. Rahnamaeian M, Cytrynska M, Zdybicka-Barabas A,Vilcinskas A. The functional interaction between abaecinand pore-forming peptides indicates a general mechanismof antibacterial potentiation. Peptides. 2016;78:17–23.

24. Saltykova ES, L’Vov AV, Ben’kovskaia GV, Poskriakov AV,Nikolenko AG. [Difference in the gene expression ofantibacterial peptides abaecin, hymenoptaecin, defensin inbees Apis mellifera and Apis mellifera caucasica]. Zh EvolBiokhim Fiziol. 2005;41:404–407.

25. Phan TT, Nguyen HD, Schumann W. Development of a strongintracellular expression system for Bacillus subtilis byoptimizing promoter elements. J Biotechnol. 2012;157:167–172.

26. Philibert T, Rao Z, Yang T, et al. Heterologous expression andcharacterization of a new heme-catalase in Bacillus subtilis168. J Ind Microbiol Biotechnol. 2016;43:729–740.

27. Pozsgai ER, Blair KM, Kearns DB. Modified marinertransposons for random inducible-expression insertions andtranscriptional reporter fusion insertions in Bacillus subtilis.Appl Environ Microbiol. 2012;78:778–785.

28. Rubinstein SM, Kolodkin-Gal I, McLoon A, et al. Osmoticpressure can regulate matrix gene expression in Bacillussubtilis. Mol Microbiol. 2012;86:426–436.

29. Zhang L, Li X, Wei D, Wang J, Shan A, Li Z. Expression ofplectasin in Bacillus subtilis using SUMO technology by amaltose-inducible vector. J Ind Microbiol Biotechnol.2015;42:1369–1376.

30. Serrano M, Gao J, Bota J, et al. Dual-specificity anti-sigmafactor reinforces control of cell-type specific gene expressionin Bacillus subtilis. PLoS Genet. 2015;11:e1005104.

31. Song W, Nie Y, Mu XQ, Xu Y. Enhancement of extracellularexpression of Bacillus naganoensis pullulanase fromrecombinant Bacillus subtilis: effects of promoter and host.Protein Expr Purif. 2016;124:23–31.

32. Summpunn P, Chaijan S, Isarangkul D, Wiyakrutta S,Meevootisom V. Characterization, gene cloning, andheterologous expression of beta-mannanase from athermophilic Bacillus subtilis. J Microbiol. 2011;49:86–93.