Characterization of an Escherichia coli rff mutant defective in transfer of N-acetylmannosaminuronic acid (ManNAcA) from UDP-ManNAcA to a lipid-linked intermediate involved in enterobacterial common antigen synthesis

An exopolysaccharide pathway from a freshwater Sphingomonas isolate

Bacteria embellish their cell envelopes with a variety of specialized polysaccharides. Biosynthesis pathways for these glycans are complex, and final products vary greatly in their chemical structures, physical properties, and biological activities. This tremendous diversity comes from the ability to arrange complex pools of monosaccharide building blocks into polymers with many possible linkage configurations. Due to the complex chemistry of bacterial glycans, very few biosynthetic pathways have been defined in detail. As part of an initiative to characterize novel polysaccharide biosynthesis enzymes, we isolated a bacterium from Lake Michigan called Sphingomonas sp. LM7 that is proficient in exopolysaccharide (EPS) production. We identified genes that contribute to EPS biosynthesis in LM7 by screening a transposon mutant library for colonies displaying altered colony morphology. A gene cluster was identified that appears to encode a complete wzy/wzx-dependent polysaccharide assembly pathway. Deleting individual genes in this cluster caused a non-mucoid phenotype and a corresponding loss of EPS secretion, confirming the role of this gene cluster in polysaccharide production. We extracted EPS from LM7 cultures and determined that it contains a linear chain of 3- and 4-linked glucose, galactose, and glucuronic acid residues. Finally, we show that the EPS pathway in Sphingomonas sp. LM7 diverges from that of sphingan-family EPSs and adhesive polysaccharides such as the holdfast that are present in other Alphaproteobacteria. Our approach of characterizing complete biosynthetic pathways holds promise for engineering polysaccharides with valuable properties.

IMPORTANCE

Bacteria produce complex polysaccharides that serve a range of biological functions. These polymers often have properties that make them attractive for industrial applications, but they remain woefully underutilized. In this work, we studied a novel polysaccharide called promonan that is produced by Sphingomonas sp. LM7, a bacterium we isolated from Lake Michigan. We extracted promonan from LM7 cultures and identified which sugars are present in the polymer. We also identified the genes responsible for polysaccharide production. Comparing the promonan genes to those of other bacteria showed that promonan is distinct from previously characterized polysaccharides. We conclude by discussing how the promonan pathway could be used to produce new polysaccharides through genetic engineering.

Structures, functions, and syntheses of glycero-glycophospholipids

Biological membranes consist of integral and peripheral protein-associated lipid bilayers. Although constituent lipids vary among cells, membrane lipids are mainly classified as phospholipids, glycolipids, and sterols. Phospholipids are further divided into glycerophospholipids and sphingophospholipids, whereas glycolipids are further classified as glyceroglycolipids and sphingoglycolipids. Both glycerophospholipids and glyceroglycolipids contain diacylglycerol as the common backbone, but their head groups differ. Most glycerolipids have polar head groups containing phosphate esters or sugar moieties. However, trace components termed glycero-glycophospholipids, each possessing both a phosphate ester and a sugar moiety, exist in membranes. Recently, the unique biological activities of glycero-glycophospholipids have attracted considerable attention. In this review, we describe the structure, distribution, function, biosynthesis, and chemical synthetic approaches of representative glycero-glycophospholipids-phosphatidylglucoside (PtdGlc) and enterobacterial common antigen (ECA). In addition, we introduce our recent studies on the rare glycero-glyco"pyrophospho"lipid, membrane protein integrase (MPIase), which is involved in protein translocation across biomembranes.

Key contributions of a glycolipid to membrane protein integration

Regulation of membrane protein integration involves molecular devices such as Sec-translocons or the insertase YidC. We have identified an integration-promoting factor in the inner membrane of Escherichia coli called membrane protein integrase (MPIase). Structural analysis revealed that, despite its enzyme-like name, MPIase is a glycolipid with a long glycan comprising N-acetyl amino sugars, a pyrophosphate linker, and a diacylglycerol (DAG) anchor. Additionally, we found that DAG, a minor membrane component, blocks spontaneous integration. In this review, we demonstrate how they contribute to Sec-independent membrane protein integration in bacteria using a comprehensive approach including synthetic chemistry and biophysical analyses. DAG blocks unfavorable spontaneous integrations by suppressing mobility in the membrane core, whereas MPIase compensates for this. Moreover, MPIase plays critical roles in capturing a substrate protein to prevent its aggregation, attracting it to the membrane surface, facilitating its insertion into the membrane, and delivering it to other factors. The combination of DAG and MPIase efficiently regulates the integration of membrane proteins.

A novel exopolysaccharide pathway from a freshwater Sphingomonas isolate

Bacteria embellish their cell envelopes with a variety of specialized polysaccharides. Biosynthesis pathways for these glycans are complex, and final products vary greatly in their chemical structures, physical properties and biological activities. This tremendous diversity comes from the ability to arrange complex pools of monosaccharide building blocks into polymers with many possible linkage configurations. Due to the complex chemistry of bacterial glycans, very few biosynthetic pathways have been defined in detail. To better understand the breadth of polysaccharide production in nature we isolated a bacterium from Lake Michigan called Sphingomonas sp. LM7 that is proficient in exopolysaccharide (EPS) production. We identified genes that contribute to EPS biosynthesis in LM7 by screening a transposon mutant library for colonies displaying altered colony morphology. A gene cluster was identified that appears to encode a complete wzy/wzx-dependent polysaccharide assembly pathway. Deleting individual genes in this cluster caused a non-mucoid phenotype and a corresponding loss of EPS secretion, confirming that LM7 assembles a novel wzy/wzx-dependent polysaccharide. We extracted EPS from LM7 cultures and showed that it contains a linear chain of 3- and 4- linked glucose, galactose, and glucuronic acid residues. Finally, we found that the EPS pathway we identified diverges from those of adhesive polysaccharides such as the holdfast that are conserved in higher Alphaproteobacteria. Our approach of characterizing complete biosynthetic pathways holds promise for engineering of polysaccharides with valuable properties.

O antigen biogenesis sensitises Escherichia coli K-12 to bile salts, providing a plausible explanation for its evolutionary loss

Escherichia coli K-12 is a model organism for bacteriology and has served as a workhorse for molecular biology and biochemistry for over a century since its first isolation in 1922. However, Escherichia coli K-12 strains are phenotypically devoid of an O antigen (OAg) since early reports in the scientific literature. Recent studies have reported the presence of independent mutations that abolish OAg repeating-unit (RU) biogenesis in E. coli K-12 strains from the same original source, suggesting unknown evolutionary forces have selected for inactivation of OAg biogenesis during the early propagation of K-12. Here, we show for the first time that restoration of OAg in E. coli K-12 strain MG1655 synergistically sensitises bacteria to vancomycin with bile salts (VBS). Suppressor mutants surviving lethal doses of VBS primarily contained disruptions in OAg biogenesis. We present data supporting a model where the transient presence and accumulation of lipid-linked OAg intermediates in the periplasmic leaflet of the inner membrane interfere with peptidoglycan sacculus biosynthesis, causing growth defects that are synergistically enhanced by bile salts. Lastly, we demonstrate that continuous bile salt exposure of OAg-producing MG1655 in the laboratory, can recreate a scenario where OAg disruption is selected for as an evolutionary fitness benefit. Our work thus provides a plausible explanation for the long-held mystery of the selective pressure that may have led to the loss of OAg biogenesis in E. coli K-12; this opens new avenues for exploring long-standing questions on the intricate network coordinating the synthesis of different cell envelope components in Gram-negative bacteria.

Subcellular localization of the enterobacterial common antigen GT-E-like glycosyltransferase, WecG

Enterobacterales have developed a specialized outer membrane polysaccharide (enterobacterial common antigen [ECA]). ECA biosynthesis begins on the cytoplasmic side of the inner membrane (IM) where glycosyltransferases sequentially add sugar moieties to form a complete repeat unit which is then translocated across the IM by WzxE before being polymerized into short linear chains by WzyE/WzzE. Research into WecG, the enzyme responsible for generating ECA lipid-II, has not progressed beyond Barr et al. (1988) who described WecG as a membrane protein. Here we revise our understanding of WecG and re-characterize it as a peripherally associated membrane protein. Through the use of Western immunoblotting we show that WecG in Shigella flexneri is maintained to the IM via its three C-terminal helices and further identify key residues in helix II which are critical for this interaction which has allowed us to identify WecG as a GT-E glycosyltransferase. We investigate the possibility of protein complexes and ultimately show that ECA lipid-I maintains WecG to the membrane which is crucial for its function. This research is the first since Barr et al. (1988) to investigate the biochemistry of WecG and reveals possible novel drug targets to inhibit WecG and thus ECA function and cell viability.

wecB Gene of Salmonella Gallinarum Plays a Critical Role in Systemic Infection of Fowl Typhoid

Salmonella enterica serovar Gallinarum (S. Gallinarum) is a host-specific pathogen causing fowl typhoid, a severe systemic infection in poultry, which leads to substantial economic losses due to high morbidity and mortality in many developing countries. However, less is known about the pathogenic characteristics and mechanism of S. Gallinarum-induced systemic infection in chickens. In this study, we deleted the S. Gallinarum UDP-N-acetylglucosamine-1-phosphate transferase gene, which contributes to the biosynthesis of enterobacterial common antigen (ECA), and studied the pathogenicity of this wecB::Cm strain in a chicken model of systemic infection. The wecB::Cm mutant strain showed comparable growth but lower resistance to bile acid and nalidixic acid than the wild-type strain in vitro. In the oral infection model of chickens, the virulence of the wecB::Cm strain was significantly attenuated in vivo. Chickens infected with wild-type strain showed typical clinical signs and pathological changes of fowl typhoid and died between 6 and 9 days post-infection, and the bacteria rapidly disseminated to systemic organs and increased in the livers and spleens. In contrast, the wecB::Cm mutant strain did not cause chicken death, there were no significant clinical changes, and the bacterial numbers in the liver and spleen of the chickens were significantly lower than those of the chickens infected with the wild-type strain. In addition, the expression of interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and CXCLi1 in the livers of wecB::Cm-infected chickens was significantly lower than that of the chickens infected with the wild-type strain. Furthermore, the attenuated wecB::Cm strain could persistently colonize the liver and spleen at low levels for up to 25 days post-infection and could induce a protective immune response in the chickens. These results indicate that the wecB gene is an important virulence factor of S. Gallinarum in the chicken model of systemic infection, and the avirulent wecB::Cm mutant could possibly be used as a live-attenuated vaccine strain for controlling fowl typhoid.

Capsular polysaccharide from a fish-gut bacterium induces/promotes apoptosis of colon cancer cells in vitro through Caspases' pathway activation

Among the widespread malignancies colorectal cancer is the most lethal. Treatments of this malignant tumor include surgery for lesions and metastases, radiotherapy, immunotherapy, and chemotherapy. Nevertheless, novel therapies to reduce morbidity and mortality are demanding. Natural products, such as polysaccharides, can be a valuable alternative to sometimes very toxic chemotherapeutical agents, also because they are biocompatible and biodegradable biomaterials. Microbial polysaccharides have been demonstrated to fulfill this requirement. In this paper, the results about the structure and the activity of a capsular polysaccharide isolated from the psychrotroph Pseudoalteromonas nigrifaciens Sq02-Rif^r, newly isolated from the fish intestine, have been described. The characterization has been obtained by spectroscopic and chemical methods, and it is supported by the bioinformatic analysis. The polymer activates Caspases 3 and 9 on colon cancer cells CaCo-2 and HCT-116, indicating a promising antitumor effect, and suggesting a potential capacity of CPS to induce apoptosis.

Enterobacterial Common Antigen: Synthesis and Function of an Enigmatic Molecule

The outer membrane (OM) of Gram-negative bacteria poses a barrier to antibiotic entry due to its high impermeability. Thus, there is an urgent need to study the function and biogenesis of the OM. In Enterobacterales, an order of bacteria with many pathogenic members, one of the components of the OM is enterobacterial common antigen (ECA). We have known of the presence of ECA on the cell surface of Enterobacterales for many years, but its properties have only more recently begun to be unraveled. ECA is a carbohydrate antigen built of repeating units of three amino sugars, the structure of which is conserved throughout Enterobacterales. There are three forms of ECA, two of which (ECAPG and ECALPS) are located on the cell surface, while one (ECACYC) is located in the periplasm. Awareness of the importance of ECA has increased due to studies of its function that show it plays a vital role in bacterial physiology and interaction with the environment. Here, we review the discovery of ECA, the pathways for the biosynthesis of ECA, and the interactions of its various forms. In addition, we consider the role of ECA in the host immune response, as well as its potential roles in host-pathogen interaction. Furthermore, we explore recent work that offers insights into the cellular function of ECA. This review provides a glimpse of the biological significance of this enigmatic molecule.

A cell cycle and nutritional checkpoint controlling bacterial surface adhesion

In natural environments, bacteria often adhere to surfaces where they form complex multicellular communities. Surface adherence is determined by the biochemical composition of the cell envelope. We describe a novel regulatory mechanism by which the bacterium, Caulobacter crescentus, integrates cell cycle and nutritional signals to control development of an adhesive envelope structure known as the holdfast. Specifically, we have discovered a 68-residue protein inhibitor of holdfast development (HfiA) that directly targets a conserved glycolipid glycosyltransferase required for holdfast production (HfsJ). Multiple cell cycle regulators associate with the hfiA and hfsJ promoters and control their expression, temporally constraining holdfast development to the late stages of G1. HfiA further functions as part of a 'nutritional override' system that decouples holdfast development from the cell cycle in response to nutritional cues. This control mechanism can limit surface adhesion in nutritionally sub-optimal environments without affecting cell cycle progression. We conclude that post-translational regulation of cell envelope enzymes by small proteins like HfiA may provide a general means to modulate the surface properties of bacterial cells.

The modality of enterobacterial common antigen polysaccharide chain lengths is regulated by o349 of the wec gene cluster of Escherichia coli K-12

The assembly of the phosphoglyceride-linked form of enterobacterial common antigen (ECA(PG)) occurs by a mechanism that involves modulation of polysaccharide chain length. However, the genetic determinant of this modulation has not been identified. Site-directed mutagenesis of o349 of the Escherichia coli K-12 wec gene cluster revealed that this locus encodes a Wzz protein that specifically modulates the chain length of ECA(PG) polysaccharides, and we have designated this locus wzz(ECA). The Wzz(ECA)-mediated modulation of ECA(PG) polysaccharide chains is the first demonstrated example of Wzz regulation involving a polysaccharide that is not linked to the core-lipid A structure of lipopolysaccharide.

Role of the rfe gene in the synthesis of the O8 antigen in Escherichia coli K-12

The Escherichia coli O8 antigen is a mannan composed of the trisaccharide repeat unit -->3)-alpha-Man-(1-->2)-alpha-Man-(1-->2)-alpha-Man-(1--> (K. Reske and K. Jann, Eur. J. Biochem. 67:53-56, 1972), and synthesis of the O8 antigen is rfe dependent (G. Schmidt, H. Mayer, and P. H. Mäkelä, J. Bacteriol. 127:755-762, 1976). The rfe gene has recently been identified as encoding a tunicamycin-sensitive UDP-GlcNAc:undecaprenylphosphate GlcNAc-1-phosphate transferase (U. Meier-Dieter, K. Barr, R. Starman, L. Hatch, and P. D. Rick, J. Biol. Chem. 267:746-753, 1992). However, the role of rfe in O8 side chain synthesis is not understood. Thus, the role of the rfe gene in the synthesis of the O8 antigen was investigated in an rfbO8+ (rfb genes encoding O8 antigen) derivative of E. coli K-12 mutant possessing a defective phosphoglucose isomerase (pgi). The in vivo synthesis of O8 side chains was inhibited by the antibiotic tunicamycin. In addition, putative lipid carrier-linked O8 side chains accumulated in vivo when lipopolysaccharide outer core synthesis was precluded by growing cells in the absence of exogenously supplied glucose. The lipid carrier-linked O8 antigen was extracted from cells and treated with mild acid in order to release free O8 side chains. The water-soluble O8 side chains were then purified by affinity chromatography using Sepharose-bound concanavalin A. Characterization of the affinity-purified O8 side chains revealed the occurrence of glucosamine in the reducing terminal position of the polysaccharide chains. The data presented suggest that GlcNAc-pyrophosphorylundecaprenol functions as the acceptor of mannose residues for the in vivo synthesis of O8 side chains in E. coli K-12.

Functions of the gene products of Escherichia coli

A list of currently identified gene products of Escherichia coli is given, together with a bibliography that provides pointers to the literature on each gene product. A scheme to categorize cellular functions is used to classify the gene products of E. coli so far identified. A count shows that the numbers of genes concerned with small-molecule metabolism are on the same order as the numbers concerned with macromolecule biosynthesis and degradation. One large category is the category of tRNAs and their synthetases. Another is the category of transport elements. The categories of cell structure and cellular processes other than metabolism are smaller. Other subjects discussed are the occurrence in the E. coli genome of redundant pairs and groups of genes of identical or closely similar function, as well as variation in the degree of density of genetic information in different parts of the genome.

Cloning and expression of the rfe-rff gene cluster of Escherichia coli

We have cloned a 13 kb Escherichia coli DNA fragment which complemented the rfe mutation to recover the biosynthesis of E. coli O9 polysaccharide. Using Tn5 insertion inactivation, the rfe gene was localized at the 1.5 kb HindIII-EcoRI region flanking the rho gene. We constructed an rfe-deficient E. coli K-12 mutant by site-directed inactivation using a DNA fragment of the cloned 1.5 kb rfe gene. This also confirmed the presence of the rfe gene in the 1.5 kb region. By simultaneous introduction of both the rfe plasmid and the plasmid of our previously cloned E. coli O9 rfb into this rfe mutant, we succeeded in achieving in vivo reconstitution of O9 polysaccharide biosynthesis. From sequence analysis of the rfe gene, a putative promoter followed by an open reading frame (ORF) was identified downstream of the rho gene. This ORF coincided with the position of the rfe gene determined by Tn5 analysis and site-directed mutagenesis. Furthermore, we identified the rff genes in the 10.5 kb DNA flanking the rfe gene. We recognized at least two functional domains on this cloned rff region. Region I complemented a newly found K-12 rff mutant, A238, to synthesize the enterobacterial common antigen (ECA). Deletion of region II resulted in the synthesis of ECAs with shorter sugar chains. When the 10.5 kb rff genes of the plasmid were inactivated by either deletion or Tn5 insertion, the plasmid lost its ability to give rise to transformants of the rfe mutants.

Biosynthesis of the Escherichia coli K5 polysaccharide, a representative of group II capsular polysaccharides: polymerization in vitro and characterization of the product

Biosynthesis of the capsular K5 polysaccharide of Escherichia coli, which has the structure 4)-beta GlcA-1,4-alpha GlcNAc-(1, was studied with membrane preparations from an E. coli K5 wild-type strain and from a recombinant K-12 strain expressing the K5 capsule. Polymerization occurs at the inner face of the cytoplasmic membrane without the participation of lipid-linked oligosaccharides. The serological K5 specificity of the in vitro product was determined with a K5-specific monoclonal antibody in an antigen-binding assay. The K5 polysaccharide, as obtained from the membranes after an in vitro incubation, has 2-keto-3-deoxyoctulosonic acid as the reducing sugar, which indicates that the polysaccharide grows by chain elongation at the nonreducing end.

Localization of enterobacterial common antigen immunoreactivity in the ribosomal cytoplasm of Escherichia coli cells cryosubstituted and embedded at low temperature

The application of two on-section immunogold labeling techniques, the Lowicryl K4M (progressive lowering of temperature) procedure and the cryosection technique of Tokuyasu, in a previous work to study the topology of enterobacterial common antigen (ECA) biosynthesis revealed the presence of label on the outer membrane and in areas associated with the inner side of the cytoplasmic membrane. However, labeling was also observed in the ribosomal cytoplasm. The question of whether the cytoplasmic label was a result of ECA displacement during the more slowly acting aldehyde fixation or whether cytoplasmic ECA precursors are true constituents of the ribosomal cytoplasm could not be resolved from these results. In the study described here, cells of the same Escherichia coli F470 strain were reinvestigated by comparison of the progressive lowering of temperature and improved cryosubstitution-low-temperature embedment techniques. The latter procedure, applied directly to nonpretreated and noncentrifuged cells, led to superior ultrastructural preservation of the cytoplasmic organization, with little opportunity for cytoplasmic antigen displacement after the primary cryofixation step; the label distribution obtained supports the conclusion that N-acetylmannosaminuronic acid (ManNAcA)-containing ECA precursors are real constituents of the ribosomal cytoplasm. Results from tunicamycin inhibition studies of ECA biogenesis in the E. coli mutant 2465 suggested that even the ECA precursor UDP-ManNAcA alone or a chemically unidentified product(s) generated from accumulated ManNAcA residues may react with the monoclonal antibody used, leading to weak but clearly positive cytoplasmic labeling. The relatively intense labeling obtained with cells grown in the absence of the drug can be explained by the reactivity of further ManNAcA-containing ECA precursors with the monoclonal antibody used.

In vitro synthesis of a lipid-linked trisaccharide involved in synthesis of enterobacterial common antigen

The heteropolysaccharide chains of enterobacterial common antigen (ECA) are made up of linear trisaccharide repeat units with the structure----3)-alpha-D-Fuc4NAc-(1----4)- beta-D-ManNAcA-(1----4)-alpha-D-GlcNAc-(1----, where Fuc4NAc is 4-acetamido-4,6-dideoxy-D-galactose, ManNAcA is N-acetyl-D-mannosaminuronic acid, and GlcNAc is N-acetyl-D-glucosamine. The assembly of these chains involves lipid-linked intermediates, and both GlcNAc-pyrophosphorylundecaprenol (lipid I) and ManNAcA-GlcNAc-pyrophosphorylundecaprenol (lipid II) are intermediates in ECA biosynthesis. In this study we demonstrated that lipid II serves as the acceptor of Fuc4NAc residues in the assembly of the trisaccharide repeat unit of ECA chains. Incubation of Escherichia coli membranes with UDP-GlcNAc, UDP-[14C]ManNAcA, and TDP-[3H]Fuc4NAc resulted in the synthesis of a radioactive glycolipid (lipid III) that contained both [14C]ManNAcA and [3H]Fuc4NAc. The oligosaccharide moiety of lipid III was identified as a trisaccharide by gel-permeation chromatography, and the in vitro synthesis of lipid III was dependent on prior synthesis of lipids I and II. Accordingly, the incorporation of [3H]Fuc4NAc into lipid III from the donor TDP-[3H]Fuc4NAc was dependent on the presence of both UDP-GlcNAc and UDP-ManNAcA in the reaction mixtures. In addition, the in vitro synthesis of lipid III was abolished by tunicamycin. Direct conversion of lipid II to lipid III was demonstrated in two-stage reactions in which membranes were initially incubated with UDP-GlcNAc and UDP-[14C]ManNAcA to allow the synthesis of radioactive lipid II. Subsequent addition of TDP-Fuc4Nac to the washed membranes resulted in almost complete conversion of radioactive lipid II to lipid III. The in vitro synthesis of lipid III was also accompanied by the apparent utilization of this lipid intermediate for the assembly of ECA heteropolysaccharide chains. Incubation of membranes with UDP-[3H]GlcNAc, UDP-ManNAcA, and TDP-Fuc4NAc resulted in the apparent incorporation of isotope into ECA polymers, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. In addition, the in vitro incorporation of [3H]Fuc4NAc into ECA heteropolysaccharide chains was demonstrated with ether-treated cells that were prepared from delta rfbA mutants of Salmonella typhimurium. These mutants are defective in the synthesis of TDP-Fuc4NAc; as a consequence, they are also defective in the synthesis of lipid III and they accumulate lipid II. Accordingly, incubation of ether-permeabilized cells of delta rfbA mutants with TDP-[3h]Fuc4NAc resulted in the incorporation of isotope into both lipid III and ECA heteropolysaccharide chains.

Accumulation of a lipid-linked intermediate involved in enterobacterial common antigen synthesis in Salmonella typhimurium mutants lacking dTDP-glucose pyrophosphorylase

The heteropolysaccharide chains of enterobacterial common antigen (ECA) are composed of linear trisaccharide repeat units having the structure----3)-alpha-Fuc4NAc-(1----4)-beta-D-ManNAcA-(1---- 4)-alpha-D-GlcNAc- (1----. Mutants of Salmonella typhimurium lacking the structural gene for dTDP-glucose pyrophosphorylase (rfbA) are severely impaired in their ability to synthesize dTDP-glucose, which is a precursor of dTDP-4-acetamido-4,6-dideoxy-D-galactose (Fuc4NAc), the donor of Fuc4NAc residues for ECA synthesis. These mutants synthesize only trace amounts of ECA, and they are hypersensitive to sodium dodecyl sulfate (SDS). Incubation of delta rfbA mutants with [3H]N-acetylglucosamine ([3H]GlcNAc) resulted in the accumulation of radioactivity in N-acetyl-D-mannosaminuronic acid (ManNAcA)-GlcNAc-pyrophosphorylundecaprenol (lipid II), the putative acceptor of Fuc4NAc residues in ECA synthesis. Lipid II did not accumulate in either wild-type cells or in rff mutants unable to synthesize ManNAcA. Both the accumulation of lipid II and the synthesis of trace amounts of ECA were abolished when delta rfbA mutants were grown in the presence of the antibiotic tunicamycin. Tunicamycin also prevented the SDS-mediated lysis of the mutants. SDS-resistant derivatives of delta rfbA mutants were isolated that were no longer able to synthesize trace amounts of ECA. Characterization of these derivatives revealed that they were defective in various steps of ECA synthesis leading to the synthesis of lipid II. The data support the conclusion that accumulation of lipid II is responsible in some way for the hypersensitivity of delta rfbA mutants to SDS.