Role of a sugar-lipid intermediate in colanic acid synthesis by Escherichia coli

ElyC and Cyclic Enterobacterial Common Antigen Regulate Synthesis of Phosphoglyceride-Linked Enterobacterial Common Antigen

The Gram-negative cell envelope is a complex structure delineating the cell from its environment. Recently, we found that enterobacterial common antigen (ECA) plays a role maintaining the outer membrane (OM) permeability barrier, which excludes toxic molecules including many antibiotics. ECA is a conserved carbohydrate found throughout Enterobacterales (e.g., Salmonella, Klebsiella, and Yersinia). There are two OM forms of ECA (phosphoglyceride-linked ECA_PG and lipopolysaccharide-linked ECA_LPS) and one periplasmic form of ECA (cyclic ECA_CYC). ECA_PG, found in the outer leaflet of the OM, consists of a linear ECA oligomer attached to phosphoglyceride through a phosphodiester linkage. The process through which ECA_PG is produced from polymerized ECA is unknown. Therefore, we set out to identify genes interacting genetically with ECA_PG biosynthesis in Escherichia coli K-12 using the competition between ECA and peptidoglycan biosynthesis. Through transposon-directed insertion sequencing, we identified an interaction between elyC and ECA_PG biosynthesis. ElyC is an inner membrane protein previously shown to alter peptidoglycan biosynthesis rates. We found ΔelyC was lethal specifically in strains producing ECA_PG without other ECA forms, suggesting ECA_PG biosynthesis impairment or dysregulation. Further characterization suggested ElyC inhibits ECA_PG synthesis in a posttranscriptional manner. Moreover, the full impact of ElyC on ECA levels requires the presence of ECA_CYC. Our data demonstrate ECA_CYC can regulate ECA_PG synthesis in strains wild type for elyC. Overall, our data demonstrate ElyC and ECA_CYC act in a novel pathway that regulates the production of ECA_PG, supporting a model in which ElyC provides feedback regulation of ECA_PG production based on the periplasmic levels of ECA_CYC.

Tracking Colanic Acid Repeat Unit Formation from Stepwise Biosynthesis Inactivation in Escherichia coli

Colanic acid is a glycopolymer loosely associated with the outer membrane of Escherichia coli that plays a role in pathogen survival. For nearly six decades since its discovery, the functional identities of the enzymes necessary to synthesize colanic acid have yet to be assessed in full. Herein, we developed a method for detecting the lipid-linked intermediates from each step of colanic acid biosynthesis in E. coli. The accumulation of each enzyme product was made possible by inactivating sequential genes involved in colanic acid biosynthesis and upregulating the colanic acid operon by inducing rcsA transcription. LC-MS analysis revealed that these accumulated materials were consistent with the well-documented composition analysis. Recapitulating the native bioassembly of colanic acid enabled us to identify the functional roles of the last two enzymes, WcaL and WcaK, associated with the formation of the lipid-linked oligosaccharide repeating unit of colanic acid. Importantly, biochemical evidence is provided for the formation of the final glycosylation hexasaccharide product formed by WcaL and the addition of a pyruvate moiety to form a pyruvylated hexasaccharide by WcaK. These findings provide insight into the development of methods for the identification of enzyme functions during cell envelope synthesis.

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.

Colanic acid biosynthesis in Escherichia coli is dependent on lipopolysaccharide structure and glucose availability

Lipopolysaccharide and colanic acid are important forms of exopolysaccharides located on the cell surface of Escherichia coli, but their interrelation with the cell stress response is not well understood. In this study, nine mutant strains with different structures of lipopolysaccharide were constructed from E. coli MG1655 by deletion of a single gene or multiple genes. All mutant strains did not produce colanic acid when grown in LB medium, but six of them could produce colanic acid when grown either in M9 medium in which glucose is the sole carbon source or in LB medium supplemented with glucose. The results indicate that colanic acid production in E. coli is dependent on both lipopolysaccharide structure and glucose availability. However, transcriptional analysis showed that 20 genes related to the colanic acid biosynthesis and the key gene rcsA in the Rcs system were all transcriptionally up-regulated in all of the nine mutant strains no matter they were grown in M9 or LB medium. This suggests that the availability of some nucleotide-sugar precursors shared by the biosynthesis of lipopolysaccharide and colanic acid might play a major role in colanic acid production in E. coli. Lipopolysaccharide pathway might have a huge priority to colanic acid pathway to use the common precursors; therefore, the colanic acid is not produced in MG1655 and the nine mutants when grown in LB medium. In the six mutant strains that can produce colanic acid in the glucose rich media, the common precursors might be abundant because they were not needed for synthesizing the mutant lipopolysaccharide.

Bacterial exopolysaccharides: biosynthesis pathways and engineering strategies

Bacteria produce a wide range of exopolysaccharides which are synthesized via different biosynthesis pathways. The genes responsible for synthesis are often clustered within the genome of the respective production organism. A better understanding of the fundamental processes involved in exopolysaccharide biosynthesis and the regulation of these processes is critical toward genetic, metabolic and protein-engineering approaches to produce tailor-made polymers. These designer polymers will exhibit superior material properties targeting medical and industrial applications. Exploiting the natural design space for production of a variety of biopolymer will open up a range of new applications. Here, we summarize the key aspects of microbial exopolysaccharide biosynthesis and highlight the latest engineering approaches toward the production of tailor-made variants with the potential to be used as valuable renewable and high-performance products for medical and industrial applications.

Bacterial cell wall synthesis gene uppP is required for Burkholderia colonization of the Stinkbug Gut

To establish a host-bacterium symbiotic association, a number of factors involved in symbiosis must operate in a coordinated manner. In insects, bacterial factors for symbiosis have been poorly characterized at the molecular and biochemical levels, since many symbionts have not yet been cultured or are as yet genetically intractable. Recently, the symbiotic association between a stinkbug, Riptortus pedestris, and its beneficial gut bacterium, Burkholderia sp., has emerged as a promising experimental model system, providing opportunities to study insect symbiosis using genetically manipulated symbiotic bacteria. Here, in search of bacterial symbiotic factors, we targeted cell wall components of the Burkholderia symbiont by disruption of uppP gene, which encodes undecaprenyl pyrophosphate phosphatase involved in biosynthesis of various bacterial cell wall components. Under culture conditions, the ΔuppP mutant showed higher susceptibility to lysozyme than the wild-type strain, indicating impaired integrity of peptidoglycan of the mutant. When administered to the host insect, the ΔuppP mutant failed to establish normal symbiotic association: the bacterial cells reached to the symbiotic midgut but neither proliferated nor persisted there. Transformation of the ΔuppP mutant with uppP-encoding plasmid complemented these phenotypic defects: lysozyme susceptibility in vitro was restored, and normal infection and proliferation in the midgut symbiotic organ were observed in vivo. The ΔuppP mutant also exhibited susceptibility to hypotonic, hypertonic, and centrifugal stresses. These results suggest that peptidoglycan cell wall integrity is a stress resistance factor relevant to the successful colonization of the stinkbug midgut by Burkholderia symbiont.

Protein-protein interactions among the components of the biosynthetic machinery responsible for exopolysaccharide production in Streptococcus thermophilus MR-1C

AIM

This study identified protein-protein interactions among the biosynthetic machinery responsible for exopolysaccharide (EPS) production in Streptococcus thermophilus MR-1C.

METHODS AND RESULTS

Protein-protein interactions were investigated using the yeast two-hybrid system. A strong protein-protein interaction was detected between the transmembrane activation protein Wzd and the protein tyrosine kinase Wze. Weaker protein-protein interactions were detected between two duplicate Wze proteins and between Wze and the phosphotyrosine phosphatase Wzh. Protein-protein interactions involving a Wzd/Wze fusion protein and Wzd and Wze may indicate that these proteins form multi-protein complexes. All combinations of the Wzh, Wzd, Wze, Wzg (regulation), CpsE (glycosyl-1-phosphate transferase), CpsS (polymerization), CpsL (unknown), CpsW (regulation) and CpsU (membrane translocation) were analysed for protein-protein interactions but no additional interactions were discovered using the yeast two-hybrid system.

CONCLUSIONS

Interactions among the phosphotyrosine phosphatase, tyrosine kinase, and transmembrane activation protein are important in the regulation of capsule biosynthesis in Strep. thermophilus MR-1C.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides some valuable insight into the organization and interactions between the many proteins involved in EPS production. A better understanding of this process may facilitate the genetic manipulation of capsule production to impart desirable properties to dairy starter cultures.

The N-acetylmannosamine transferase catalyzes the first committed step of teichoic acid assembly in Bacillus subtilis and Staphylococcus aureus

There have been considerable strides made in the characterization of the dispensability of teichoic acid biosynthesis genes in recent years. A notable omission thus far has been an early gene in teichoic acid synthesis encoding the N-acetylmannosamine transferase (tagA in Bacillus subtilis; tarA in Staphylococcus aureus), which adds N-acetylmannosamine to complete the synthesis of undecaprenol pyrophosphate-linked disaccharide. Here, we show that the N-acetylmannosamine transferases are dispensable for growth in vitro, making this biosynthetic enzyme the last dispensable gene in the pathway, suggesting that tagA (or tarA) encodes the first committed step in wall teichoic acid synthesis.

Substrate specificity and membrane topology of Escherichia coli PgpB, an undecaprenyl pyrophosphate phosphatase

The synthesis of the lipid carrier undecaprenyl phosphate (C(55)-P) requires the dephosphorylation of its precursor, undecaprenyl pyrophosphate (C(55)-PP). The latter lipid is synthesized de novo in the cytosol and is also regenerated after its release from the C(55)-PP-linked glycans in the periplasm. In Escherichia coli the dephosphorylation of C(55)-PP was shown to involve four integral membrane proteins, BacA, and three members of the type 2 phosphatidic acid phosphatase family, PgpB, YbjG, and YeiU. Here, the PgpB protein was purified to homogeneity, and its phosphatase activity was examined. This enzyme was shown to catalyze the dephosphorylation of C(55)-PP with a relatively low efficiency compared with diacylglycerol pyrophosphate and farnesyl pyrophosphate (C(15)-PP) lipid substrates. However, the in vitro C(55)-PP phosphatase activity of PgpB was specifically enhanced by different phospholipids. We hypothesize that the phospholipids are important determinants to ensure proper conformation of the atypical long axis C(55) carrier lipid in membranes. Furthermore, a topological analysis demonstrated that PgpB contains six transmembrane segments, a large periplasmic loop, and the type 2 phosphatidic acid phosphatase signature residues at a periplasmic location.

The biosynthesis of peptidoglycan lipid-linked intermediates

The biosynthesis of bacterial cell wall peptidoglycan is a complex process involving many different steps taking place in the cytoplasm (synthesis of the nucleotide precursors) and on the inner and outer sides of the cytoplasmic membrane (assembly and polymerization of the disaccharide-peptide monomer unit, respectively). This review summarizes the current knowledge on the membrane steps leading to the formation of the lipid II intermediate, i.e. the substrate of the polymerization reactions. It makes the point on past and recent data that have significantly contributed to the understanding of the biosynthesis of undecaprenyl phosphate, the carrier lipid required for the anchoring of the peptidoglycan hydrophilic units in the membrane, and to the characterization of the MraY and MurG enzymes which catalyze the successive transfers of the N-acetylmuramoyl-peptide and N-acetylglucosamine moieties onto the carrier lipid, respectively. Enzyme inhibitors and antibacterial compounds interfering with these essential metabolic steps and interesting targets are presented.

Periplasmic phosphorylation of lipid A is linked to the synthesis of undecaprenyl phosphate

One-third of the lipid A found in the Escherichia coli outer membrane contains an unsubstituted diphosphate unit at position 1 (lipid A 1-diphosphate). We now report an inner membrane enzyme, LpxT (YeiU), which specifically transfers a phosphate group to lipid A, forming the 1-diphosphate species. (32)P-labelled lipid A obtained from lpxT mutants do not produce lipid A 1-diphosphate. In vitro assays with Kdo(2)-[4'-(32)P]lipid A as the acceptor shows that LpxT uses undecaprenyl pyrophosphate as the substrate donor. Inhibition of lipid A 1-diphosphate formation in wild-type bacteria was demonstrated by sequestering undecaprenyl pyrophosphate with the cyclic polypeptide antibiotic bacitracin, providing evidence that undecaprenyl pyrophosphate serves as the donor substrate within whole bacteria. LpxT-catalysed phosphorylation is dependent upon transport of lipid A across the inner membrane by MsbA, a lipid A flippase, indicating a periplasmic active site. In conclusion, we demonstrate a novel pathway in the periplasmic modification of lipid A that is directly linked to the synthesis of undecaprenyl phosphate, an essential carrier lipid required for the synthesis of various bacterial polymers, such as peptidoglycan.

Colicin M Exerts Its Bacteriolytic Effect via Enzymatic Degradation of Undecaprenyl Phosphate-linked Peptidoglycan Precursors

Colicin M was earlier demonstrated to provoke Escherichia coli cell lysis via inhibition of cell wall peptidoglycan (murein) biosynthesis. As the formation of the O-antigen moiety of lipopolysaccharides was concomitantly blocked, it was hypothesized that the metabolism of undecaprenyl phosphate, an essential carrier lipid shared by these two pathways, should be the target of this colicin. However, the exact target and mechanism of action of colicin M was unknown. Colicin M was now purified to near homogeneity, and its effects on cell wall peptidoglycan metabolism reinvestigated. It is demonstrated that colicin M exhibits both in vitro and in vivo enzymatic properties of degradation of lipid I and lipid II peptidoglycan intermediates. Free undecaprenol and either 1-pyrophospho-MurNAc-pentapeptide or 1-pyrophospho-MurNAc-(pentapeptide)-Glc-NAc were identified as the lipid I and lipid II degradation products, respectively, showing that the cleavage occurred between the lipid moiety and the pyrophosphoryl group. This is the first time such an activity is described. Neither undecaprenyl pyrophosphate nor the peptidoglycan nucleotide precursors were substrates of colicin M, indicating that both undecaprenyl and sugar moieties were essential for activity. The bacteriolytic effect of colicin M therefore appears to be the consequence of an arrest of peptidoglycan polymerization steps provoked by enzymatic degradation of the undecaprenyl phosphate-linked peptidoglycan precursors.

CpsE from type 2 Streptococcus pneumoniae catalyzes the reversible addition of glucose-1-phosphate to a polyprenyl phosphate acceptor, initiating type 2 capsule repeat unit formation

The majority of the 90 capsule types made by the gram-positive pathogen Streptococcus pneumoniae are assembled by a block-type mechanism similar to that utilized by the Wzy-dependent O antigens and capsules of gram-negative bacteria. In this mechanism, initiation of repeat unit formation occurs by the transfer of a sugar to a lipid acceptor. In S. pneumoniae, this step is catalyzed by CpsE, a protein conserved among the majority of capsule types. Membranes from S. pneumoniae type 2 strain D39 and Escherichia coli containing recombinant Cps2E catalyzed incorporation of [14C]Glc from UDP-[14C]Glc into a lipid fraction in a Cps2E-dependent manner. The Cps2E-dependent glycolipid product from both membranes was sensitive to mild acid hydrolysis, suggesting that Cps2E was catalyzing the formation of a polyprenyl pyrophosphate Glc. Addition of exogenous polyprenyl phosphates ranging in size from 35 to 105 carbons to D39 and E. coli membranes stimulated Cps2E activity. The stimulation was due, in part, to utilization of the exogenous polyprenyl phosphates as an acceptor. The glycolipid product synthesized in the absence of exogenous polyprenyl phosphates comigrated with a 60-carbon polyprenyl pyrophosphate Glc. When 10 or 100 microM UMP was added to reaction mixtures containing D39 membranes, Cps2E activity was inhibited 40% and 80%, respectively. UMP, which acted as a competitive inhibitor of UDP-Glc, also stimulated Cps2E to catalyze the reverse reaction, with synthesis of UDP-Glc from the polyprenyl pyrophosphate Glc. These data indicated that Cps2E was catalyzing the addition of Glc-1-P to a polyprenyl phosphate acceptor, likely undecaprenyl phosphate.

Gene products required for surface expression of the capsular form of the group 1 K antigen in Escherichia coli (O9a:K30)

The group 1 K30 antigen from Escherichia coli (O9a:K30) is present on the cell surface as both a capsular structure composed of high-molecular-weight K30 polysaccharide and as short K30 oligosaccharides linked to lipid A-core in a lipopolysaccharide molecule (K30LPS). To determine the molecular processes that are responsible for the two forms of K antigen, the 16 kb chromosomal cps region has been characterized. This region encodes 12 gene products required for the synthesis, polymerization and translocation of the K30 antigen. The gene products include four glycosyltransferases responsible for synthesis of the K30 repeat unit; a PST (1) exporter (Wzx), required to transfer lipid-linked K30 units across the plasma membrane to the periplasmic space; and a K30-antigen polymerase (Wzy). These gene products are typical of those seen in O-antigen biosynthesis gene clusters and they interact with the lipopolysaccharide translocation pathway to express K30LPS on the cell surface. The same gene products also provide the biosynthetic intermediates for the capsule assembly pathway, although they are not in themselves sufficient for synthesis of the K30 capsule. Three additional genes, wza, wzb and wzc, encode homologues to proteins that are encoded by gene clusters involved in expression of a variety of bacterial exopolysaccharides. Mutant analysis indicates that Wza and Wzc are required for wild-type surface expression of the capsular structure but are not essential for polymerization and play no role in the translocation of K30LPS. These surface expression components provide the key feature that distinguishes the assembly systems for O antigens and capsules.

Assignment of biochemical functions to glycosyl transferase genes which are essential for biosynthesis of exopolysaccharides in Sphingomonas strain S88 and Rhizobium leguminosarum

Glycosyl transferases which recognize identical substrates (nucleotide-sugars and lipid-linked carbohydrates) can substitute for one another in bacterial polysaccharide biosynthesis, even if the enzymes originate in different genera of bacteria. This substitution can be used to identify the substrate specificities of uncharacterized transferase genes. The spsK gene of Sphingomonas strain S88 and the pssDE genes of Rhizobium leguminosarum were identified as encoding glucuronosyl-(B1-->4)-glucosyl transferases based on reciprocal genetic complementation of mutations in the spsK gene and the pssDE genes by segments of cloned DNA and by the SpsK-dependent incorporation of radioactive glucose (Glc) and glucuronic acid (GlcA) into lipid-linked disaccharides in EDTA-permeabilized cells. By contrast, glycosyl transferases which form alternative sugar linkages to the same substrate caused inhibition of polysaccharide synthesis or were deleterious or lethal in a foreign host. The negative effects also suggested specific substrate requirements: we propose that spsL codes for a glucosyl-(beta1-->4)-glucuronosyl transferase in Sphingomonas and that pssC codes for a glucuronosyl-(beta1-->4)-glucuronosyl transferase in R. leguminosarum. Finally, the complementation results indicate the order of attachment of sphingan main-chain sugars to the C55-isoprenylphosphate carrier as -Glc-GlcA-Glc-isoprenylpyrophosphate.

Organization of the Escherichia coli K-12 gene cluster responsible for production of the extracellular polysaccharide colanic acid

Colanic acid (CA) is an extracellular polysaccharide produced by most Escherichia coli strains as well as by other species of the family Enterobacteriaceae. We have determined the sequence of a 23-kb segment of the E. coli K-12 chromosome which includes the cluster of genes necessary for production of CA. The CA cluster comprises 19 genes. Two other sequenced genes (orf1.3 and galF), which are situated between the CA cluster and the O-antigen cluster, were shown to be unnecessary for CA production. The CA cluster includes genes for synthesis of GDP-L-fucose, one of the precursors of CA, and the gene for one of the enzymes in this pathway (GDP-D-mannose 4,6-dehydratase) was identified by biochemical assay. Six of the inferred proteins show sequence similarity to glycosyl transferases, and two others have sequence similarity to acetyl transferases. Another gene (wzx) is predicted to encode a protein with multiple transmembrane segments and may function in export of the CA repeat unit from the cytoplasm into the periplasm in a process analogous to O-unit export. The first three genes of the cluster are predicted to encode an outer membrane lipoprotein, a phosphatase, and an inner membrane protein with an ATP-binding domain. Since homologs of these genes are found in other extracellular polysaccharide gene clusters, they may have a common function, such as export of polysaccharide from the cell.

Regulation of expression of group IA capsular polysaccharides in Escherichia coli and related extracellular polysaccharides in other bacteria

Bacterial surface polysaccharides fulfill a number of important roles in cell-cell interactions, survival in natural environments, and formation of biofilms. Consequently, the mechanisms involved in regulation of extracellular polysaccharides are predicted to have a significant impact on microbial adaptation. Strains of Escherichia coli, Klebsiella spp, and Erwinia spp produce extracellular polysaccharides which share structural features. There are also similarities in the organization of genes required for synthesis of these cell surface polymers and, in some cases, the mechanism of synthesis may be related. Despite the diverse habitats of these bacteria, the systems which regulate expression of their extracellular polysaccharides appear to share components and mechanisms. Understanding these regulatory processes may lead to novel therapeutic approaches for pathogens, or for control of unwanted biofilm formation in industrial settings.

Mechanism of bacitracin resistance in gram-negative bacteria that synthesize exopolysaccharides

Four representative species from three genera of gram-negative bacteria that secrete exopolysaccharides acquired resistance to the antibiotic bacitracin by stopping synthesis of the exopolysaccharide. Xanthomonas campestris, Sphingomonas strains S-88 and NW11, and Escherichia coli K-12 secrete xanthan gum, sphingans S-88 and NW11, and colanic acid, respectively. The gumD gene in X. campestris is required to attach glucose-P to C55-isoprenyl phosphate, the first step in the assembly of xanthan. A recombinant plasmid carrying the gumD gene of X. campestris restored polysaccharide synthesis to bacitracin-resistant exopolysaccharide-negative mutants of X. campestris and Sphingomonas strains. Similarly, a newly cloned gene (spsB) from strain S-88 restored xanthan synthesis to the same X. campestris mutants. However, the intergeneric complementation did not extend to mutants of E. coli that were both resistant to bacitracin and nonproducers of colanic acid. The genetic results also suggest mechanisms for assembling the sphingans which have commercial potential as gelling and viscosifying agents.

Coexpression of colanic acid and serotype-specific capsular polysaccharides in Escherichia coli strains with group II K antigens

In Escherichia coli K-12, the rcsA and rcsB gene products are positive regulators in expression of the slime polysaccharide colanic acid. We have previously demonstrated the presence of rcsA sequences in E. coli K1 and K5, strains with group II capsular K antigens, and shown that introduction of multicopy rcsA into these strains results in the expression of colanic acid. We report here the presence of rcsB sequences in E. coli K1 and K5 and demonstrate that RcsB also plays a role in the biosynthesis of colanic acid in strains with group II K antigens. In E. coli K1 and K5 grown at 37 degrees C, multicopy rcsB and the resulting induction of colanic acid synthesis had no significant effect on synthesis of the group II K antigens. K-antigen-specific sugar transferase activities were not significantly different in the presence or absence of multicopy rcsB, and introduction of a cps mutation to eliminate colanic acid biosynthesis in a K1-derivative strain did not influence the activity of the polysialyltransferase enzyme responsible for synthesis of the K1 polymer. Furthermore, immunoelectron microscopy showed no detectable difference in the size or distribution of the group II K-antigen capsular layer in cells which produced colanic acid. Colanic acid expression therefore does not appear to significantly affect synthesis of the group II K-antigen capsule and, unlike for group I K antigens, expression of group II K antigens is not positively regulated by the rcs system.

The biology of colicin M

This communication summarizes our present knowledge of colicin M, an unusual member of the colicin group. The gene encoding colicin M, cma, has been sequenced and the protein isolated and purified. With a deduced molecular size of 29,453 Da, colicin M is the smallest of the known colicins. The polypeptide can be divided into functional domains for cell surface receptor binding, uptake into the cell, and killing activity. To kill, the colicin must enter from outside the cell. Colicin M blocks the biosynthesis of both peptidoglycan and O-antigen by inhibiting regeneration of the bactoprenyl-P carrier lipid. Autolysis occurs as a secondary effect following inhibition of peptidoglycan synthesis. Colicin M is the only colicin known to have such a mechanism of action. Immunity to this colicin is mediated by the cmi gene product, a protein of 13,890 Da. This cytoplasmic membrane protein confers immunity by binding to and thus neutralizing the colicin. Cmi shares properties with both immunity proteins of the pore-forming and the cytoplasmically active colicins. Genes for the colicin and immunity protein are found next to each other, but in opposite orientation, on pColM plasmids. The mechanism of colicin M release is not known.

Biosynthesis of a D-glucosyl polyisoprenyl diphosphate in particulate preparations of Micrococcus lysodeikticus

Particulate fractions of Micrococcus lysodeikticus incubated with UDP-D-[14C]glucose incorporated radioactivity into a chloroform - methanol-soluble, low-mol. wt. compound, and into a polymer. The low-mol. wt. compound consisted of a glucolipid that was extremely labile to mild acid hydrolysis with the formation of D-[14C]glucose, and to mild alkali, yielding 14C-labeled alpha-D-glucopyranose 1,2-phosphate and D-glucose 2-phosphate. The labeled glucolipid was eluted from a DEAE-cellulose column at a salt concentration higher than that required by synthetic ficaprenyl (D-glucopyranosyl phosphate), and it migrated more slowly than the latter compound in t.l.c. Formation of the glucolipid was stimulated by exogenous ficaprenyl phosphate, but not by C55-dolichyl phosphate. These results suggest that the [14C]glucolipid has the characteristic properties of a polyisoprenyl glucosyl diphosphate.

Cloned DNA fragment specifying major outer membrane protein a in Escherichia coli K-12

Plasmid pMC44 is a recombinant plasmid that contains a 2-megadalton EcoRI fragment of Escherichia coli K-12 DNA joined to the cloning vehicle, pSC101. The polypeptides specified by plasmid pMC44 were identified and compared with those specified by pSC101 to determine those that are unique to pMC44. Three polypeptides specified by plasmid pMC44 were localized in the cell envelope fraction of minicells: a Sarkosyl-insoluble outer membrane polypeptide (designated M2), specified by the cloned 2-megadalton DNA fragment, and two Sarkosyl-soluble membrane polypeptides specified by the cloning plasmid pSC101. Bacteria containing plasmid pMC44 synthesized quantities of M2 approximately equal to the most abundant E. coli K-12 outer membrane protein. Evidence is presented that outer membrane polypeptide M2, specified by the recombinant plasmid pMC44, is the normal E. coli outer membrane protein designated protein a by Lugtenberg and 3b by Schnaitman.