(Received for publication, May 2, 1997, and in revised form, May 29, 1997)
From the Department of Biochemistry, Duke University
Medical Center, Durham, North Carolina 27710, the ¶ Middle
Atlantic Mass Spectrometry Laboratory, Department of Pharmacology, The
Johns Hopkins University School of Medicine,
Baltimore, Maryland 21205-2185, the
Laboratory of Vaccine
Development and Immune Mechanisms, National Institute of Public Health
and the Environment, 3720 BA Bilthoven, The Netherlands, and ** The
Centre for Veterinary Science, Department of Clinical Veterinary
Medicine, University of Cambridge, Madingley Road,
Cambridge CB3 0ES, United Kingdom
The first reaction of lipid A biosynthesis in
Gram-negative bacteria is catalyzed by
UDP-N-acetylglucosamine (UDP-GlcNAc) O-acyltransferase, the product of the lpxA
gene. The reaction involves the transfer of an acyl chain from
hydroxyacyl-acyl carrier protein (ACP) to the glucosamine 3-OH position
of UDP-GlcNAc. The lipid A isolated from Escherichia coli
contains (R)-3-hydroxymyristate at the 3 and 3 positions.
Accordingly, LpxA of E. coli is highly selective for
(R)-3-hydroxymyristoyl-ACP over ACP thioesters of longer or shorter acyl chains. We now demonstrate that the
lpxA gene from Neisseria meningitidis encodes a
similar acyltransferase that selectively utilizes 3-hydroxylauroyl-ACP.
Strains of E. coli harboring the temperature-sensitive
lpxA2 mutation make very little lipid A and lose viability
rapidly at 42 °C. We have created an E. coli strain in
which the chromosomal lpxA2 mutation is complemented by the
N. meningitidis lpxA gene introduced on a plasmid. This strain, RO138/pTO6, grows similarly to wild type cells at 42 °C and
produces wild type levels of lipid A. However, the lipid A isolated
from RO138/pTO6 contains mostly hydroxylaurate and hydroxydecanoate in
the 3 and 3
positions. The strain RO138/pTO6 is more susceptible than
wild type to certain antibiotics at 42 °C. This is the first report
of an E. coli strain growing with shortened hydroxyacyl chains on its lipid A. The lpxA gene product appears to be
a critical determinant of the length of the ester-linked hydroxyacyl
chains found on lipid A in living cells.
Lipopolysaccharide
(LPS)1 is the major component
of the outer leaflet of the outer membrane of Gram-negative bacteria
(1, 2). Intact LPS is known to be important in maintaining the permeability barrier of the outer membrane (1-3). The lipid anchor portion of LPS, lipid A, is required for bacterial growth and is
responsible for some of the toxicity of Gram-negative bacteria to
mammalian hosts (1, 4-6). Lipid A is a -1
,6-linked glucosamine disaccharide substituted with (R)-3-hydroxyacyl groups at
the 2, 3, 2
, and 3
positions and phosphates at the 1 and 4
positions (1, 5, 7-9). Lipid A of E. coli is additionally esterified at the 2
and 3
(R)-3-hydroxymyristoyl groups with laurate
and myristate, respectively (1). The first enzyme in the lipid A
biosynthetic pathway, LpxA (1, 7, 10), catalyzes the transfer of an
(R)-3-hydroxy fatty acid from hydroxyacyl-acyl carrier
protein (ACP) to the glucosamine 3-OH position of
UDP-N-acetylglucosamine (UDP-GlcNAc) (11-16). LpxA is thus
responsible for attachment of the O-linked fatty acids
located at the 3 and 3
positions of mature lipid A (Fig.
1) (1, 7, 10). The lengths of these fatty
acids differ among bacterial species (7, 9, 16); the lipid A molecules
of Pseudomonas aeruginosa (17) and Rhodobacter sphaeroides (18) are substituted with hydroxydecanoate at the 3 and 3
positions; the lipid A from strains of Neisseria
(19-21) is substituted with hydroxylaurate, and bacteria of the family Enterobacteriaceae (8, 9, 16) contain lipid A molecules substituted with hydroxymyristate. The UDP-GlcNAc acyltransferases found in cell extracts of some of these bacteria display high specificity for acyl-ACP thioesters of the same acyl chain lengths that
predominate in their lipid As in vivo (16).
Thus far, only conditional mutants of genes encoding the early lipid A biosynthetic enzymes have been identified, indicating that lipid A is required for growth (1, 10). One lpxA mutation, lpxA2, has been extensively investigated (22, 23). Strains carrying this mutation are temperature-sensitive and have little measurable LpxA activity at any temperature, as judged by assays of cell extracts. Mutants harboring lpxA2 also are characterized by a very low lipid A to glycerophospholipid ratio compared with wild type cells at 42 °C (22, 23). In addition, such mutants have been shown to be supersusceptible to many antibiotics, perhaps due to the effect of the low lipid A content on outer membrane integrity (24, 25). These features reinforce the idea that LpxA and certain other enzymes of LPS biosynthesis are targets for the design of new antibiotics (26).
Escherichia coli lipid A activates the synthesis of
cytokines and various other inflammatory mediators in macrophages (1, 5, 27). However, the lipid A molecules from some other organisms, for
instance R. sphaeroides, are potent antagonists of that
response in mouse and human cells (28-30). One major difference in the
lipid A of R. sphaeroides, as compared with E. coli, is that the 3 and 3 O-linked fatty acids are
hydroxydecanoates rather than hydroxymyristates (18, 30). Thus, subtle
features of the structure of lipid A, especially the nature of the
fatty acids at the ester-linked positions, appear to be important in
triggering the activation of macrophages.
We now report that the LpxA from Neisseria meningitidis catalyzes the transfer of 3-hydroxylaurate from 3-hydroxylauroyl-ACP to the glucosamine 3-OH position of UDP-GlcNAc, as predicted from the N. meningitidis lipid A structure (20, 21). To determine whether a foreign LpxA selective for a different fatty acid could substitute for the enzyme of wild type E. coli, we have introduced the N. meningitidis lpxA gene (31) on a plasmid into RO138, an E. coli strain harboring the lpxA2 mutation (23). The resulting strain, RO138/pTO6, has wild type growth characteristics. However, the lipid A isolated from RO138/pTO6 is extensively substituted with shorter than normal ester-linked hydroxy fatty acids. This new strain provides a system in which to investigate the effects of a specific lipid A modification on E. coli growth and outer membrane function.
[-32P]UTP and
32Pi were purchased from NEN Life Science
Products. Tryptone, yeast extract, and agar were from Difco.
Antibiotics, glucosamine 1-phosphate, acyl carrier protein,
(R,S)-3-hydroxydecanoic acid,
(R,S)-3-hydroxylauric acid, and
(R,S)-3-hydroxymyristic acid were products of Sigma.
Chloroform, methanol, acetic acid, pyridine, and 88% formic acid were
from Mallinckrodt Chemical Works. All other chemicals were obtained
from Sigma or Mallinckrodt. Silica gel-60 thin layer plates (0.25 mm)
were purchased from E. Merck, Darmstadt, Germany. Restriction enzymes
and T4 DNA ligase were from New England Biolabs or Boehringer Mannheim.
Shrimp alkaline phosphatase was from U. S. Biochemical Corp.
Primers for PCR and sequencing were custom-made by Life Technologies,
Inc. The LpxC inhibitor L-573,655 (26) was provided by Dr. A. Patchett
(Merck Research Laboratories). The FabA inhibitor
3-decynoyl-N-acetylcysteamine (33) was provided by Dr. John
Cronan, University of Illinois, Urbana.
The bacterial
strains used in this study are all derivatives of E. coli
K-12. DH5F
was from our laboratory collection; XL1 Blue MR was
purchased from Stratagene, and BL21(DE3) and BL21(DE3)pLysS were from
Novagen. SM105 (lpxA+, Strepr) has been
described previously (22). RO138 (lpxA2 recA,
Strepr) was obtained from Matt Anderson, Merck Research
Laboratories. Cells were grown in LB medium (34), containing 10 g/liter
tryptone, 5 g/liter yeast extract, and 10 g/liter NaCl, or on LB plus
15 g/liter agar, at either 30, 37, or 42 °C, as indicated.
Antibiotics were used, when appropriate, at the following final
concentrations: 100 µg/ml ampicillin, 20 µg/ml chloramphenicol, and
30 µg/ml streptomycin.
Most recombinant DNA
techniques were as described by Sambrook et al. (35).
Plasmids were prepared using the Wizard Plus Miniprep DNA Purification
System (Promega) or the Bigger Prep Plasmid DNA Preparation Kit (5 Prime 3 Prime, Inc., Boulder, CO). Restriction endonucleases, T4
DNA ligase, and shrimp alkaline phosphatase were used according to the
manufacturers' specifications. DNA was extracted from gels using the
GeneClean kit (Bio 101), according to the manufacturer's
directions.
Cells were made competent for transformation by CaCl2 treatment (35). Transformants were plated on LB agar containing appropriate antibiotics and incubated at 30 or 37 °C.
Plasmid ConstructionsThe plasmids used in this study are
listed in Table I. We chose to use the
pET expression system (Novagen), in which genes are driven by the T7
promoter, to obtain high levels of protein expression. The N. meningitidis lpxA gene (31) was cloned for expression in E. coli as follows. PCR primers were designed from the DNA sequence
of N. meningitidis lpxA (31) to generate LpxA protein
carrying a C-terminal fusion of six histidine residues (a His-Tag) to
enable one-step purification of the expressed protein. NdeI
and HindIII sites were incorporated into these
oligonucleotide primers to facilitate cloning. The primer sequences
were as follows: forward primer 5 GAA AGG AAA CAT ATG ACC
CTC 3
and reverse primer 5
GGG GTT TCA AGC TTC AGT GAT
GGT GAT GGT GAT GGC GGA TGA TGC CGC GCG C 3
. NdeI and
HindIII sites are underlined. Using N. meningitidis MC58 chromosomal DNA as template, a PCR reaction was
carried out with forward and reverse primers, using Pfu
polymerase (Stratagene). The cycle was 90 °C for 2 min, 55 °C for
2 min, and 72 °C for 2 min. This cycle was repeated 30 times. The
PCR reaction was electrophoresed on a 0.8% agarose gel, and the
expected DNA species of approximately 800 bp was identified. DNA from
this fragment was eluted using GeneClean and subsequently digested with
NdeI and HindIII. The digested DNA fragment was
ligated into pET-21a(+) that had been cut with NdeI and
HindIII. The ligation mixture was transformed into E. coli XLI-Blue cells with selection on 100 µg/ml ampicillin.
Resultant colonies were picked and grown for DNA preparation. Positive
recombinants were screened by digestion with NdeI and
HindIII to excise the 800-bp insert. This led to the
identification of pNMHT (His-Tag LpxA) plasmid.
|
The E. coli lpxA gene (14, 36) was amplified using the
following primers: 5 CCG CCG GGA ATT CCA TAT
GAT TGA TAA ATC CGC CT 3
and 5
CGC GGA TCC TGT TCA
GTC ATT AAC GAA TCA GAC CG 3
. The upstream and downstream primers
introduced NdeI and BamHI sites (underlined),
respectively. Plamsid pSR1 (15) was used as the template. The PCR
reaction mixture included 5 ng of template, 0.5 µM each
primer, 10 mM KCl, 10 mM
(NH4)2SO4, 20 mM
Tris-Cl (pH 8.75), 2 mM MgSO4, 0.1% Triton
X-100, 100 µg/ml bovine serum albumin, 4 mM
MgCl2, 200 µM each dNTP (Boehringer
Mannheim), and 2.5 units of Pfu polymerase (Stratagene) in a
final volume of 0.1 ml. This mixture was subjected to a 5-min
denaturation at 94 °C and then 20 cycles of [94 °C for 30 s, 55 °C for 30 s, 72 °C for 30 s] plus a 7-min extension
at 72 °C in a Perkin-Elmer GeneAmp PCR System 2400. The PCR product
was purified by gel electrophoresis. Simultaneously, pBluescriptKS(+)
was digested with PvuII, and the 2514-bp piece was purified
by gel electrophoresis. The 3
ends of the vector and PCR product were
then "tailed" with dGTP and dCTP, respectively, by treatment with
50 units of terminal transferase (Boehringer Mannheim), according to
the manufacturer's specifications. These pieces were ligated together
and transformed into DH5
F
with selection on ampicillin. Plasmids
from resultant colonies were screened for the presence of the correct
insert by digestion with NdeI and BamHI. One
correct plasmid preparation was digested with NdeI and
BamHI, and the resulting insert (containing the E. coli lpxA gene) was ligated into the similarly cut and phosphatase-treated vector, pET23c. The ligation mixture was
transformed into DH5
F
with selection on ampicillin, and plasmids
from resultant colonies were screened for the presence of the correct
insert by digestion with NdeI and HindIII. One
correct plasmid, designated pTO1, was re-transformed into DH5
F
for
amplification and preparation, and finally transformed into
BL21(DE3)pLysS. The sequence of the entire lpxA gene in pTO1
was confirmed using primers based on the published sequence (36) and a
DNA sequencing kit from U. S. Biochemical Corp.
We chose the vector pNGH1 (graciously provided by Nora Gardner Haigh, Department of Biochemistry, Duke University) for low level expression of the E. coli and N. meningitidis lpxA genes in SM105 and RO138. pNGH1 was constructed by ligation of the 1505-bp BamHI-SphI fragment from pTrc99A (Pharmacia Biotech Inc.) to the 4057-bp BamHI-SphI fragment from pACYC184 (New England Biolabs), yielding a low copy number vector containing the IPTG-inducible trp-lac promoter and the lacIq repressor gene, and conferring chloramphenicol resistance. Plasmid pNGH1 was transformed into SM105 and RO138 as a vector control in the experiments involving pTO6 and pTO7.
The first step in constructing pTO6 was to ligate the 800-bp XbaI-HindIII fragment from pNMHT into the similarly cut and phosphatase-treated vector pBluescriptSK(+), to give the plasmid pTO4. This plasmid was constructed to obtain flanking restriction sites suitable for subcloning into pNGH1. Subsequently, the 898-bp SacI-KpnI fragment from pTO4 was ligated into the similarly cut and phosphatase-treated vector pNGH1. The ligation mixture was transformed into XL1-Blue MR with selection on chloramphenicol, and plasmids from resultant colonies were screened for the presence of the correct insert by digestion with SacI and KpnI. One correct plasmid, designated pTO6, was retransformed into XL1-Blue MR for amplification and preparation. This plasmid was then transformed into SM105 and RO138.
Plasmid pTO7 was constructed similarly. The 800-bp XbaI-HindIII fragment from pTO1 was ligated into the similarly cut and phosphatase-treated vector pBluescriptSK(+), to give plasmid pTO5. Then the 877-bp SacI-SacI fragment of pTO5 was ligated into the similarly cut and phosphatase-treated vector pNGH1. The ligation mixture was transformed into XL1-Blue MR with selection on chloramphenicol, and plasmids from the resultant colonies were screened for the presence of the correct insert by digestion with SacI. One correct plasmid, designated pTO7, was transformed into XL1-Blue MR for amplification and preparation. This plasmid was then transformed into SM105 and RO138.
Preparation of Cell ExtractsStrains BL21(DE3)/pNMHT and
BL21(DE3)pLysS/pTO1 were grown in 100 ml of LB medium containing either
ampicillin or ampicillin and chloramphenicol, as appropriate, at
37 °C to an A600 of 0.6, induced with a final
concentration of 1 mM IPTG, and grown for 3 more h. Strains
SM105/pNGH1, SM105/pTO6, SM105/pTO7, RO138/pNGH1, RO138/pTO6, and
RO138/pTO7 were grown in 100 ml of LB containing chloramphenicol at
30 °C to an A600 of 0.6, shifted to 42 °C, and grown for 3 more h. These cultures were not induced with IPTG, because there is sufficient lpxA expression without
induction. Cultures were harvested by centrifugation at 4000 × g for 15 min. Cells were washed once with 10 ml of 10 mM potassium phosphate (pH 7), 0.2 M NaCl, and
20% glycerol, and resuspended in 3-5 ml of the same buffer. Cells
were broken by one passage through a French pressure cell at 18,000 p.s.i. This suspension was then centrifuged at 10,000 × g for 20 min, and the supernatant was stored at 80 °C.
Protein concentration was determined using the bicinchoninic acid assay
(Pierce) (37).
[-32P]UDP-GlcNAc was prepared as described
previously (38) with minor modifications. The initial reaction was
allowed to proceed for only 1 h at room temperature, and the
acetylation reaction was not quenched by boiling. In addition, the
triethyl ammonium bicarbonate buffer was used at pH 8.5, and the final product was dried in a SpeedVac (Savant), rather than being lyophilized (39). None of these modifications had any effect on yield.
(R,S)-3-Hydroxylauroyl-ACP and
(R,S)-3-hydroxymyristoyl-ACP were prepared as described
previously (40) with a few modifications. The acyl substrates for the
reaction were (R,S)-3-hydroxylauric acid and
(R,S)-3-hydroxymyristic acid. The acylation reactions were
allowed to proceed for 4 h at room temperature. In addition, 0.7 mM 3-decynoyl-N-acetylcysteamine, an inhibitor
of FabA (33), was added to the hydroxylauroyl-ACP acylation reaction.
(R,S)-3-Hydroxydecanoyl-ACP was prepared similarly and was a
kind gift from Dr. Garry Dotson.
This assay monitors the conversion
of [-32P]UDP-GlcNAc to
[
-32P]UDP-3-O-((R)-3-hydroxymyristoyl)-GlcNAc
(12, 13). The standard reaction mixture contained 40 mM
HEPES (pH 8), 1 mg/ml bovine serum albumin, 10 µM acyl
donor, 10 µM [
-32P]UDP-GlcNAc (2 × 106 cpm/nmol), and 100-500 µg/ml cell extract in a final
volume of 10-20 µl. In addition, 0.2 mg/ml L-573,655 (26), from a 10 mg/ml stock in dimethyl sulfoxide, was added to inhibit the LpxC
catalyzed deacetylation of the product and to prevent further
metabolism (Fig. 1). The final (2%) concentration of dimethyl
sulfoxide has no affect on LpxA activity (data not shown). The acyl
donors were (R,S)-3-hydroxy acyl thioesters of ACP. The
reaction mixture was incubated at 30 °C for 2 to 10 min (12, 13),
and reactions were terminated by spotting 2.5 µl on a silica thin
layer chromatography plate. The plates were developed in a solvent
system containing chloroform/methanol/water/acetic acid (25:15:4:2,
v/v). The plates were then exposed to imaging screens overnight at room
temperature. The plates were visualized, and the extent of the reaction
was quantified using a Molecular Dynamics PhosphorImager, operated with
ImageQuant software.
Bacteria from an overnight culture were inoculated into 3-5 ml of fresh LB medium to an A600 of 0.1, grown at 30 °C to an A600 of 0.6, and shifted to 42 °C. After 1 h, 5 µCi/ml 32Pi was added to the cultures, and the cultures were grown at 42 °C for 30 min. At this point 0.8-ml samples were removed, and lipid A and glycerophospholipids were prepared and quantified by the method of Galloway and Raetz (22) with the following modifications. Lipid A and glycerophospholipid samples were counted in Econoscint scintillation fluid and analyzed by TLC in a system containing chloroform/pyridine/formic acid/water (50:50:16:5, v/v). The plates were visualized, and the percent of the total counts attributable to the desired material was determined using a PhosphorImager.
Preparation of bis-Phospho-lipid ACultures (250 ml) of
RO138/pTO6 and RO138/pTO7 were grown with shaking (250 rpm) in LB at
30 °C to an A600 of 0.6, shifted to 42 °C,
and grown for 3 more h. Cells were harvested by centrifugation at
4000 × g for 20 min, washed once with 40 ml of
phosphate-buffered saline (35), resuspended in 80 ml of
phosphate-buffered saline, and frozen at 80 °C.
Glycerophospholipids were extracted for 1 h at room temperature
using a single phase Bligh-Dyer mixture (chloroform/methanol/aqueous
solution, 1:2:0.8, v/v), generated by addition of 100 ml of chloroform
and 200 ml of methanol to 80 ml of washed cells in phosphate-buffered
saline. The precipitated LPS, protein, and nucleic acids were collected
by centrifugation at 4000 × g for 15 min and washed
once with 250 ml of a fresh single-phase Bligh-Dyer mixture
(chloroform/methanol/water, 1:2:0.8, v/v). Pellets were then
resuspended in 80 ml of 12.5 mM ammonium (or sodium)
acetate (pH 4.5) and 1% SDS and incubated at 100 °C for 30 min to
cleave the glycosidic linkage between
3-deoxy-D-manno-octulosonic acid (Kdo) and lipid
A without loss of the 1 or 4
phosphates (41, 42). This solution was
converted to a two-phase Bligh-Dyer mixture (having the proportions
chloroform/methanol/water, 2:2:1.8, v/v) by addition of 89 ml of
chloroform and 89 ml of methanol. The phases were separated by
centrifugation at 4000 × g for 15 min. The upper phase
was washed once with a fresh neutral pre-equilibrated lower phase, and
then the combined lower phases, containing the lipid A and a few
impurities, were washed once with fresh pre-equilibrated upper phase.
Half of the lower phase was dried by rotary evaporation, resuspended in
10 ml of chloroform/methanol/water (2:3:1, v/v), and applied to a 1-ml
DEAE-cellulose (Whatman DE52) column, equilibrated as the acetate form
in the same solvent mixture (43). The column was washed with 4 ml of
chloroform/methanol/water (2:3:1, v/v) and eluted with 4 ml each of
chloroform/methanol/ammonium acetate (60, 120, 240, or 480 mM) in water (2:3:1, v/v), while collecting 1-ml fractions
throughout the elution. The bis-phospho-lipid A eluted with
the 240 mM ammonium acetate. These fractions were combined
and converted to neutral two-phase Bligh-Dyer proportions (chloroform/methanol/water, 2:2:1.8, v/v) by addition of chloroform and
water. The lower phase was dried under a stream of N2 and then frozen at
80 °C until being prepared for mass spectrometry. The preparation was monitored at various stages by spotting samples onto silica TLC plates, developing the plates in
chloroform/pyridine/88% formic acid/water (50:50:16:5. v/v), and
visualizing by sulfuric acid charring.
Liquid secondary ion mass spectra (LSIMS) were acquired on a Concept IH (Kratos Analytical, Manchester, UK) two-sector (EB geometry) mass spectrometer at a resolution of 1000. A 1-µl portion of sample solution in methanol/chloroform (1:2) was mixed with mono-thioglycerol or triethanolamine (Aldrich) on the tip of the probe. Analyte ions were desorbed from the matrix by an 8 keV Cs+ primary ion beam. Mass spectra were acquired by scanning the magnet in the 100-2500 a.m.u. range at a scan rate of 10 s/decade. Normally 10-20 scans were signal-averaged for each spectrum.
Mild base hydrolysis of lipid A samples was carried out by mixing lipid A solution with 30% NH4OH in a 2:1 (v/v) ratio for various periods (20 min to 1 h). Aliquots of the mixture were mass-analyzed without any further purification.
Testing Strains for Susceptibility to AntibioticsOvernight cultures of the strains were diluted to an A600 of 0.2. A cotton swab saturated with the culture was used to produce a lawn of cells by streaking across the plate in three directions, and discs of Whatman 42 filter paper (6 mm diameter) soaked in the various antibiotic solutions indicated were placed on the plates with flamed forceps (44). The discs were made to contain the following amounts of antibiotics by the application of 5 µl of an appropriately concentrated stock solution: novobiocin, 30 µg; erythromycin, 25 µg; rifampicin, 10 µg; and bacitracin, 10 units. Zones of complete growth inhibition were measured after incubation at 30 or 42 °C overnight.
The lpxA gene of N. meningitidis was recently cloned and found to code for a 258-amino acid (28 kDa) protein with high similarity to E. coli lpxA (31). Since the O-linked hydroxy fatty acids in the meningococcal lipid A are 3-hydroxylaurate (20, 21), it was postulated that the N. meningitidis LpxA would catalyze the transfer of 3-hydroxylaurate from 3-hydroxylauroyl-ACP to UDP-GlcNAc (31).
To test this hypothesis, we constructed pNMHT, which contains the N. meningitidis lpxA gene under control of the T7 promoter for use in the pET expression system (see "Experimental Procedures"). A similar construction, pTO1, was made containing the E. coli lpxA gene. After growth and induction with IPTG, the strains BL21(DE3)/pNMHT and BL21(DE3)pLysS/pTO1 both overexpress a 28-kDa protein, as judged by SDS-polyacrylamide gel electrophoresis and Coomassie staining (data not shown). Strain BL21(DE3), containing the pET23c vector alone, when grown and induced similarly, does not display overexpression of a protein of 28 kDa.
Extracts of the three strains were assayed for LpxA activity with each of the following acyl donor substrates: (R,S)-3-hydroxydecanoyl-ACP, (R,S)-3-hydroxylauroyl-ACP, and (R,S)-3-hydroxymyristoyl-ACP. The results from these assays are shown in Table II. The extract from the strain containing pET23c alone shows typical wild type levels of LpxA activity with (R,S)-3-hydroxymyristoyl-ACP at the substrate concentrations employed (12, 13) and very little activity with the other acyl donors. As expected, the preferred acyl donor for the overexpressed N. meningitidis LpxA is clearly 3-hydroxylaurate (Table II). As reported previously (16), E. coli LpxA prefers 3-hydroxymyristate, but it utilizes 3-hydroxylaurate at about 2% relative rate. The N. meningitidis LpxA shows slightly less specificity, utilizing 3-hydroxydecanoate at about 8% of the rate of 3-hydroxylaurate under standard assay conditions (Table II). N. meningitidis LpxA utilizes 3-hydroxymyristate at less than 2% of the rate of 3-hydroxylaurate.
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We are interested in
developing systems that permit the modification of the structure of
lipid A in living cells of E. coli to understand the
biological functions of lipid A. We therefore wanted to construct a
strain of E. coli in which N. meningitidis LpxA,
rather than E. coli LpxA, would be responsible for
incorporation of the 3 and 3 O-linked hydroxy fatty acids
of lipid A. As an approach to this problem, we used strain RO138, a
recA
derivative of SM101. The
temperature-sensitive E. coli mutant SM101
(bearing lpxA2) has very low levels of LpxA activity in cell
extracts and a nearly complete defect in lipid A biosynthesis at
42 °C in cells (22, 23). The recA
nature of
RO138 prevents recombination between plasmid-borne and chromosomal
lpxA genes. SM105, the isogenic wild type strain of SM101,
was also used as a control in some experiments (22, 23).
Plasmids pTO6 and pTO7 were constructed as described under "Experimental Procedures" to achieve modest overexpression of the N. meningitidis and E. coli lpxA genes, respectively, for use in complementation studies. The parent vector, pNGH1, was used as a control. We transformed SM105 and RO138 with pNGH1, pTO6, and pTO7 and first incubated the cells at 30 °C with chloramphenicol selection for the plasmids. Following purification at 30 °C, the transformants were restreaked and incubated at 42 °C. All strains grew rapidly and formed single colonies at 42 °C, except for RO138/pNGH1, which did not grow at all. The lpxA genes of both pTO6 and pTO7 therefore complement the temperature-sensitive phenotype of RO138 on plates (data not shown).
To determine whether the plasmids also prevent the loss of cell
viability observed in liquid cultures of SM101 at 42 °C (22, 23),
exponentially growing cells of each construct were shifted from 30 to
42 °C. RO138/pTO6 and RO138/pTO7 continued to grow after the
temperature shift, just like SM105/pNGH1, but the optical density of
the culture of RO138/pNGH1 began to decrease after 1 h at 42 °C
(Fig. 2), as reported previously for
SM101 (22, 23). Thus, the presence of the N. meningitidis
lpxA gene allows E. coli cells containing the
lpxA2 mutation to grow normally at high temperatures in
liquid culture.
Restoration of LpxA Activity in RO138/pTO6 and RO138/pTO7
To demonstrate that the N. meningitidis and E. coli lpxA genes carried on pTO6 and pTO7 are expressed and generate catalytically active LpxA, cell extracts of various strains were prepared and assayed (Table III). As expected, SM105/pNGH1 shows wild type levels of LpxA activity with 3-hydroxymyristoyl-ACP and very little activity with 3-hydroxylauroyl-ACP. RO138/pNGH1 displays no measurable acyltransferase activity under these conditions with either substrate, because of the lpxA2 mutation. Extracts of both SM105 and RO138 contain high levels of 3-hydroxylauroyl-ACP-dependent acyltransferase activity in the presence of pTO6, whereas extracts of the same strains carrying pTO7 overexpress 3-hydroxymyristoyl-dependent LpxA activity (Table III). The results demonstrate that functional N. meningitidis and E. coli LpxAs are being produced in these SM105- and RO138-derived constructs.
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The measurable but relatively low level of 3-hydroxylauroyl-ACP-dependent acyltransferase activity in extracts of SM105/pTO7 and RO138/pTO7 (Table III) can be attributed to the 2% residual activity that is characteristic of E. coli LpxA with that substrate (12, 16). Interestingly, strain SM105/pTO6 displays consistently lower 3-hydroxymyristoyl-ACP-dependent acyltransferase activity than does the SM105/pNGH1 control (Table III). Since LpxA is a trimer (36), this finding could be explained by the formation of inactive heterotrimers between N. meningitidis and E. coli LpxA monomers.
Lipid A to Glycerophospholipid Ratios in SM105- and RO138-derived StrainsThe death of strains carrying the lpxA2 mutation at 42 °C has been attributed to their reduced ability to make lipid A, as reflected by the lipid A to glycerophospholipid ratio in pulse-labeling studies with intact cells (22). This defect presumably reflects the low LpxA activity in these strains. Since the LpxA specific activity in RO138/pTO6 is comparable to that in SM105/pNGH1, but is directed toward 3-hydroxylaurate rather than 3-hydroxymyristate (Table III), we wanted to determine the lipid A to glycerophospholipid ratio in RO138/pTO6 grown at 42 °C.
Table IV shows that the lipid A to glycerophospholipid ratios of SM105/pNGH1 and RO138/pNGH1 match those reported previously (22). RO138/pTO6 displays a ratio very similar to wild type (SM105/pNGH1), whereas the ratio for RO138/pTO7 is slightly higher than normal. This may be due to the rather high degree of overexpression of LpxA activity in that strain (Table II). The three constructs with normal or slightly elevated ratios (Table IV) also display similar levels of total 32Pi incorporation into lipid A, as normalized to the optical density of the culture (data not shown). Therefore, the absolute amount of lipid A made in cells of RO138/pTO6 is similar to that found for SM105/pNGH1 (wild type).
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The lipid A from wild type E. coli contains
3-hydroxymyristoyl groups at the 2, 3, 2, and 3
positions. The 2
and
3
hydroxyacyl groups are further substituted with laurate and
myristate, forming acyloxyacyl linkages (Fig. 1 and Fig.
3). Since LpxA is responsible for the
incorporation of the hydroxyacyl groups at the 3 and 3
positions, we
expected that the lipid A isolated from RO138/pTO6 (complemented by the
N. meningitidis lpxA gene at 42 °C) would be derivatized
with shorter 3-hydroxy fatty acids at positions 3 and 3
.
Samples of bis-phospho-lipid A were prepared using pH 4.5 hydrolysis (41, 42) of cells of RO138/pTO6 and RO138/pTO7, as described
under "Experimental Procedures." The lipid A isolated by direct
hydrolysis of cells appears to be the same as lipid A obtained by
hydrolysis of LPS (21, 22, 45, 46). The most abundant negative ion
peaks in the LSIMS spectrum of the RO138/pTO7 lipid A are those at
m/z 1796.5, [M H]
and 1818.5, [M
2H + Na]
(Fig.
4A). These masses are consistent with the
known chemical structure of wild type E. coli
bis-phospholipid A (monoisotopic mass of free acid 1797.2) (1, 9).
Other minor ion peaks clustered around m/z 1570.4 and 1360.3 correspond to the loss of the hydroxymyristoyl (R3) or the
myristoyloxymyristoyl (R1OR1
) moieties (see
Fig. 3 for cleavages and terminology), respectively, consistent with
the fragmentation pattern expected for Escherichia-type lipid A. The fragment ion peaks at m/z 738.1, 710.1, 708.1, and 692.1 are the reducing-end fragments
0,1X1
,
Y1
,
Y1
-H2, and
Z1
(47), which are typical low
mass fragment anions of lipid A (Fig. 3) (48).
Negative ion LSIMS analysis of the RO138/pTO6 lipid A reveals a
significantly different pattern (Fig. 4B). A very weak ionic signal is found at the wild type [M H]
(m/z 1796.5). Another minor peak is found at m/z
1768.6, which is 28 a.m.u. less than the mass of the wild type
molecular anion. However, the most abundant anion peaks (interpreted
here as molecular anions [M
H]
of the
RO138/pTO6 lipid A) are those at m/z 1740.6, 1712.6, and 1684.4. These correspond to lipid A molecules that are 56, 84, and 112 a.m.u. smaller than the wild type species, respectively. In addition,
the measured masses of the fragment ions resulting from the
de-O-acylation of lipid A ([M
R3-H]
at m/z 1513.4 and 1541.6, and [M
R1OR1
-H]
at
m/z 1304.2 and 1332.4) are 28 and 56 a.m.u. less than
those of the analogous fragments of the wild type lipid A. The major fragment ion peaks resulting from the glycosidic bond cleavage in the
RO138/pTO6 lipid A (Y1
at
m/z 682.3 and 654.4) are 28 and 56 a.m.u. smaller than
the Y1
fragment ion peak in the
spectrum of the wild type lipid A. These results are consistent with
the hypothesis that bis-phospho-lipid A of RO138/pTO6 is a
mixture of four major molecular species, one with hydroxylaurates at
R1 and R3 (as defined in Fig. 3), one with
hydroxydecanoates at R1 and R3, and two species
of lipid A with one hydroxylaurate and one hydroxydecanoate at either
position. These findings are consistent with the acyl chain length
specificity of N. meningitidis LpxA, as determined by assays
of cell extracts (Table II). The small peaks (interpreted as [M
H]
) at m/z 1768.6 and 1796.5 (Fig.
4B) correspond to lipid A molecules containing one or two
hydroxymyristates, respectively, and these species probably result from
residual E. coli LpxA activity.
To confirm our hypothesis, mass spectral analyses were
performed on the above lipid A samples after mild base hydrolysis with ammonium hydroxide (see "Experimental Procedures"). Base treatment is known to remove preferentially the O-linked fatty acyl
and hydroxy fatty acyl chains from lipid A (49). The rate of hydrolysis of the hydroxy fatty acyl chains is more rapid (50, 51). The mass
spectra of the NH4OH-treated lipid A samples are shown in Fig. 5. The mass spectrum of incompletely
degraded lipid A from RO138/pTO7 (Fig. 5A), acquired after a
relatively short treatment with base (25 min), shows ion peaks
corresponding to intact lipid A (m/z 1796.6), as well as ion
peaks corresponding to partially de-O-acylated lipid A
molecules (loss of R3 at m/z 1570.5; loss of
R1OR1 at m/z 1360.1; and loss of
both R1OR1
and R3 at
m/z 1133.8). The mass of the
Y1
fragment ion remains unchanged.
Another abundant low mass fragment ion peak at m/z 484.4 is
likely to correspond to de-O-acylated Y1
fragment ion (i.e.
Y1
-R3).
The mass spectrum of incompletely base-hydrolyzed RO138/pTO6 lipid A
(Fig. 5B) is especially informative. The ion peaks
corresponding to intact lipid A are the same as in the spectrum of
RO138/pTO6 lipid A before base treatment (Fig. 4B). The same
is true for the ion peaks of partially de-O-acylated lipid A
(loss of R3 or of R1OR1) and the
Y1
fragment ion. Importantly, the
ion peaks Y1
-R3 and [M
R1OR1
-R3
H]
are
at m/z 484.3 and 1133.5, respectively, exactly the
same as for RO138/pTO7 (Fig. 5A)! The peak at
m/z 1133.5 dominates the mass spectrum after an hour of
NH4OH treatment (Fig. 5C), suggesting that it is
the final product of this hydrolysis treatment. Since the masses of
Y1
-R3 and [M
R1OR1
-R3
H]
are
conserved in the lipid A samples from RO138/pTO6 and RO138/pTO7, the
lipid A from the two strains must have identical R2,
R2
, and R4 groups. Furthermore, since both the
Y1
and [M
R1OR1
H]
ions in the spectrum of
hydrolyzed RO138/pTO6 lipid A are 28 and 56 a.m.u. less than their
analogs in the spectrum of the RO138/pTO7 lipid A, the R3
group of RO138/pTO6 must be either hydroxylaurate or hydroxydecanoate.
Similarly, since the [M
R3
H]
ions of the RO138/pTO6 lipid A are 28 and 56 a.m.u. smaller than those of the wild type species, the R1 (or R1
)
groups of the RO138/pTO6 lipid A must be two or four methylenes shorter
than on the lipid A from RO138/pTO7.
The only ambiguity in the RO138/pTO6 lipid A structure, therefore, is
whether the R1 or the R1 group is actually
shortened. Given that the N. meningitidis lpxA does appear
to cause the modification of the R3 group and that the
R1 and R3 groups are equivalent in the early
biosynthetic pathway (see Fig. 1), it seems probable that the
R1 group is the one that is modified. In addition, there is
no enzymological reason to expect that the R1
group would be modified in this strain. Taken together, these data strongly suggest
that RO138/pTO6 lipid A is a mixture of species substituted at the 3 and 3
positions with hydroxylaurate and/or hydroxydecanoate. Thus, in
RO138/pTO6, we have created a viable E. coli strain that synthesizes and tolerates a modified lipid A with shortened hydroxyacyl chains.
We examined the effects of the modified lipid A structure of RO138/pTO6 on antibiotic resistance. Because of its low lipid A level in its outer membrane, RO138 is hypersensitive to diverse antibiotics, including novobiocin, erythromycin, rifampicin, and bacitracin (24, 25). Some periplasmic proteins can even leak out of lipid A-deficient cells (52). Table V shows that neither pTO6 nor pTO7 has any effect on the antibiotic susceptibility of SM105, but both plasmids restore wild type levels of resistance to RO138 grown at 30 °C. However, at 42 °C, while cells of strain RO138/pTO7 show wild type levels of antibiotic resistance, cells of RO138/pTO6 are still significantly more susceptible to all four antibiotics. Thus, the lipid A that results from the substitution of the N. meningitidis LpxA for the E. coli enzyme does not form an ideal E. coli outer membrane.
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Inner and outer membrane proteins were prepared by selective Triton X-100 extraction of whole cell envelopes in the presence of magnesium (53). No major differences were observed by SDS-gel electrophoresis in the membrane protein profiles of any of the SM105-derived strains or in RO138/pTO6 and RO138/pTO7 (data not shown).
The enzyme LpxA is responsible for the incorporation of the 3 and
3 O-linked hydroxy fatty acids found in the lipid A of Gram-negative bacteria (1). In E. coli these ester-linked
fatty acids are (R)-3-hydroxymyristate (8, 9, 16), and in
N. meningitidis, they are (R)-3-hydroxylaurate
(20, 21). The LpxA enzymes from E. coli and other organisms
have been shown to have a high specificity for the ACP thioester of the
fatty acid found in the 3 and 3
positions of their lipid A, compared with ACP thioesters of longer or shorter hydroxyacyl chains (12, 16).
We have shown in this study that, as expected, the N. meningitidis LpxA is specific for 3-hydroxylauroyl-ACP as the acyl
donor substrate. How lpxA-encoded acyltransferases achieve
their remarkable acyl chain selectivity is unknown. However, the
crystal structure of E. coli LpxA was recently solved (36),
and further structural studies of LpxA enzymes having different
selectivity may lead to the identification of the acyl chain measuring
device in these unusual enzymes.
The availability of N. meningitidis LpxA has made possible
in vivo investigations of lipid A molecules with different
length acyl chains. The construction of an lpxA2 mutant
strain complemented by N. meningitidis lpxA on a plasmid
(RO138/pTO6) creates a system in which an E. coli strain
produces lipid A molecules with hydroxydecanoyl and hydroxylauroyl
chains at the O-linked 3 and 3 positions. In wild type
E. coli and Salmonella, over 95% of the lipid A
contains (R)-3-hydroxymyristate at the 3 and 3
positions (7, 9, 43). Studies of N. meningitidis have
revealed lipid A molecules with predominantly 3-hydroxylaurate (20, 21,
54), although some investigators note the presence of some
3-hydroxydecanoate in lipid A from strains of Neisseria (21,
55).
The lipid A from RO138/pTO6 contains approximately equal amounts of
3-hydroxydecanoate and 3-hydroxylaurate in the 3 and 3 positions. This
may be due to the relaxed selectivity toward acyl donor substrates of
N. meningitidis LpxA, which incorporates 3-hydroxydecanoate into UDP-GlcNAc in vitro at about 8% the rate with which it
incorporates 3-hydroxylaurate. Also, 3-hydroxydecanoyl-ACP is a branch
point metabolite in the biosynthesis of saturated and unsaturated fatty acids in E. coli (23, 33). The pool of
(R)-3-hydroxydecanoyl-ACP in E. coli cells may be
larger than that of (R)-3-hydroxylauroyl-ACP. If so, mass
action would favor (R)-3-hydroxydecanoyl-ACP as the substrate, and the percent of lipid A molecules containing
(R)-3-hydroxydecanoate would be larger than expected from
the kinetic analyses (Table II). The small fraction of the lipid A
molecules containing 3-hydroxymyristate (Figs. 4 and 5) would probably
be absent in a strain in which the E. coli chromosomal
lpxA gene was removed altogether and replaced by the
N. meningitidis gene.
On plates and in liquid media, at either 30 or 42 °C, the growth of
RO138/pTO6 is indistinguishable from the equivalent strain carrying the
E. coli lpxA gene (RO138/pTO7). In addition, RO138/pTO6 produces wild type levels of lipid A. These data demonstrate that all
the lipid A biosynthetic enzymes (Fig. 1) can effectively utilize
substrate analogs containing 3-hydroxylaurate and/or 3-hydroxydecanoate at the 3 and 3 positions. Only one other E. coli mutant
(msbB
) has been reported to grow with a
modified lipid A structure, in this case with a penta-acylated lipid A
(27, 56), rather than the usual hexa-acylated species (Fig. 1). Ours is
the first description of an E. coli mutant with a
modified lipid A that contains shorter, non-native hydroxyacyl
chains.
To date the only biological difference that we have found between
RO138/pTO6 and RO138/pTO7 is the higher sensitivity of RO138/pTO6 to
certain antibiotics at 42 °C. These types of antibiotics apparently cannot penetrate the outer membrane of wild type E. coli
(24, 25). Strains like RO138, with mutations in early lipid A
biosynthetic genes, are hypersensitive to these antibiotics (24, 25).
At 30 °C, the lipid A molecules of RO138/pTO6 are sufficient to
confer wild type levels of resistance, probably because some
hydroxymyristate is incorporated (Table V). However, at 42 °C,
RO138/pTO6 is significantly more susceptible to these antibiotics than
are wild type strains, possibly because the shorter acyl chains at the
3 and 3 positions of the lipid A result in a higher fluidity of the
outer membrane of RO138/pTO6, as compared with wild type. This
characteristic of RO138/pTO6 suggests that the acyl chain length of
E. coli lipid A is optimized to contribute to the protective
function of the outer membrane. It will be interesting to investigate
E. coli constructs expressing acyltransferases specific for
even shorter acyl chains, such as those of Pseudomonas or
Rhodobacter. Conversely, the effects of replacing N. meningitidis lpxA with E. coli lpxA will also be of
interest, especially with regard to N. meningitidis pathogenesis.
A possible application of E. coli strains producing modified
lipid A molecules is in the biological production of LPS antagonists. Currently, such antagonists are chemically synthesized (30, 57) or are
isolated from bacteria like R. sphaeroides and
Rhodobacter capsulatus (5, 28, 29). Enzymatic and/or
in vivo synthesis might increase efficiency, yield, and
accessibility. One antagonist currently in clinical trials (57) is
modeled after R. capsulatus lipid A, which contains
3-hydroxydecanoate moieties at the 3 and 3 positions, and only one
acyloxyacyl unit. The recently cloned P. aeruginosa lpxA
encodes an acyltransferase that is specific for 3-hydroxydecanoyl-ACP
as the acyl donor.2 This gene,
introduced on a plasmid, has been shown to correct the temperature
sensitivity of RO138.2 It seems likely that the RO138
strain carrying the P. aeruginosa lpxA will be found to
contain lipid A molecules with only 3-hydroxydecanoate in the 3 and 3
positions. In combination with an msbB mutation, such
strains (if viable) would have a lipid A structure almost identical to
that of R. sphaeroides. The determination of whether such
engineered lipid As are agonists or antagonists of LPS-induced responses in mammalian immune cells will be useful in defining the
subtle structural features that are responsible for the bioactivity of
lipid A-like molecules (5, 28-30, 32, 57, 58).
We thank Nora Gardner Haigh for providing us with pNGH1.