α-Glucan biosynthesis and the GlgE pathway in Mycobacterium tuberculosis

Characterization of Two Glycoside Hydrolases of Family GH13 and GH57, Present in a Polysaccharide Utilization Locus (PUL) of Pontibacter sp. SGAir0037

Glycogen, an α-glucan polymer serving as an energy storage compound in microorganisms, is synthesized through distinct pathways (GlgC-GlgA or GlgE pathway). Both pathways involve multiple enzymes, with a shared glycogen branching enzyme (GBE). GBEs play a pivotal role in establishing α-1,6-linkages within the glycogen structure. GBEs are also used for starch modification. Understanding how these enzymes work is interesting for both glycogen synthesis in microorganisms, as well as novel applications for starch modification. This study focuses on a putative enzyme GH13_9 GBE (PoGBE13), present in a polysaccharide utilization locus (PUL) of Pontibacter sp. SGAir0037, and related to the GlgE glycogen synthesis pathway. While the PUL of Pontibacter sp. SGAir0037 contains glycogen-degrading enzymes, the branching enzyme (PoGBE13) was also found due to genetic closeness. Characterization revealed that PoGBE13 functions as a typical branching enzyme, exhibiting a relatively high branching over non-branching (hydrolysis and α-1,4-transferase activity) ratio on linear maltooctadecaose (3.0 ± 0.4). Besides the GH13_9 GBE, a GH57 (PoGH57) enzyme was selected for characterization from the same PUL due to its undefined function. The combined action of both GH13 and GH57 enzymes suggested 4-α-glucanotransferase activity for PoGH57. The characterization of these unique enzymes related to a GlgE glycogen synthesis pathway provides a more profound understanding of their interactions and synergistic roles in glycogen synthesis and are potential enzymes for use in starch modification processes. Due to the structural similarity between glycogen and starch, PoGBE13 can potentially be used for starch modification with different applications, for example, in functional food ingredients.

Trehalose synthases from the subfamily GH13_16 involved in α-glucan biosynthesis - a focus on their maltokinase domain

The subfamily GH13_16 trehalose synthase (TreS) converts maltose to trehalose and vice versa. Typically, it consists of three domains, but it may contain a C-terminal extension exhibiting clear sequence features of a maltokinase (MaK). The present in silico study was focused on collection of naturally fused TreS-MaKs and their subsequent detailed bioinformatics analysis. Hence a set of total 3354 unique sequences was compared consisting of 1900 single TreSs, 1426 fused TreS-MaKs and 28 single MaKs. Fused TreS-MaKs were divided into five groups, namely with a standard MaK, with mutations in the maltose-binding site, of the catalytic nucleophile, of the general acid/base and of both catalytic residues. Sequence logos bearing the best conserved sequence regions were prepared for both TreSs and MaKs in an effort to find unique sequence features. In addition, linkers connecting the TreS and MaK parts in the fused enzymes were analysed. This analysis revealed that MaKs in fused enzymes have an extended N-terminal regions compared to single MaKs. Finally, the evolutionary relationships were demonstrated by phylogenetic trees of TreS parts from single TreSs and fused TreS-MaKs from the same organism as well as of single TreSs existing in multiple isoforms in the same organism.

Polyphosphate kinase-1 regulates bacterial and host metabolic pathways involved in pathogenesis of Mycobacterium tuberculosis

Inorganic polyphosphate (polyP) is primarily synthesized by Polyphosphate Kinase-1 (PPK-1) and regulates numerous cellular processes, including energy metabolism, stress adaptation, drug tolerance, and microbial pathogenesis. Here, we report that polyP interacts with acyl CoA carboxylases, enzymes involved in lipid biosynthesis in Mycobacterium tuberculosis. We show that deletion of ppk-1 in M. tuberculosis results in transcriptional and metabolic reprogramming. In comparison to the parental strain, the Δppk-1 mutant strain had reduced levels of virulence-associated lipids such as PDIMs and TDM. We also observed that polyP deficiency in M. tuberculosis is associated with enhanced phagosome-lysosome fusion in infected macrophages and attenuated growth in mice. Host RNA-seq analysis revealed decreased levels of transcripts encoding for proteins involved in either type I interferon signaling or formation of foamy macrophages in the lungs of Δppk-1 mutant-infected mice relative to parental strain-infected animals. Using target-based screening and molecular docking, we have identified raloxifene hydrochloride as a broad-spectrum PPK-1 inhibitor. We show that raloxifene hydrochloride significantly enhanced the activity of isoniazid, bedaquiline, and pretomanid against M. tuberculosis in macrophages. Additionally, raloxifene inhibited the growth of M. tuberculosis in mice. This is an in-depth study that provides mechanistic insights into the regulation of mycobacterial pathogenesis by polyP deficiency.

Surface Glucan Structures in Aeromonas spp

Aeromonas spp. are generally found in aquatic environments, although they have also been isolated from both fresh and processed food. These Gram-negative, rod-shaped bacteria are mostly infective to poikilothermic animals, although they are also considered opportunistic pathogens of both aquatic and terrestrial homeotherms, and some species have been associated with gastrointestinal and extraintestinal septicemic infections in humans. Among the different pathogenic factors associated with virulence, several cell-surface glucans have been shown to contribute to colonization and survival of Aeromonas pathogenic strains, in different hosts. Lipopolysaccharide (LPS), capsule and α-glucan structures, for instance, have been shown to play important roles in bacterial–host interactions related to pathogenesis, such as adherence, biofilm formation, or immune evasion. In addition, glycosylation of both polar and lateral flagella has been shown to be mandatory for flagella production and motility in different Aeromonas strains, and has also been associated with increased bacterial adhesion, biofilm formation, and induction of the host proinflammatory response. The main aspects of these structures are covered in this review.

Complete genome sequence of Arthrobacter sp. PAMC25564 and its comparative genome analysis for elucidating the role of CAZymes in cold adaptation

BACKGROUND

The Arthrobacter group is a known set of bacteria from cold regions, the species of which are highly likely to play diverse roles at low temperatures. However, their survival mechanisms in cold regions such as Antarctica are not yet fully understood. In this study, we compared the genomes of 16 strains within the Arthrobacter group, including strain PAMC25564, to identify genomic features that help it to survive in the cold environment.

RESULTS

Using 16 S rRNA sequence analysis, we found and identified a species of Arthrobacter isolated from cryoconite. We designated it as strain PAMC25564 and elucidated its complete genome sequence. The genome of PAMC25564 is composed of a circular chromosome of 4,170,970 bp with a GC content of 66.74 % and is predicted to include 3,829 genes of which 3,613 are protein coding, 147 are pseudogenes, 15 are rRNA coding, and 51 are tRNA coding. In addition, we provide insight into the redundancy of the genes using comparative genomics and suggest that PAMC25564 has glycogen and trehalose metabolism pathways (biosynthesis and degradation) associated with carbohydrate active enzyme (CAZymes). We also explain how the PAMC26654 produces energy in an extreme environment, wherein it utilizes polysaccharide or carbohydrate degradation as a source of energy. The genetic pattern analysis of CAZymes in cold-adapted bacteria can help to determine how they adapt and survive in such environments.

CONCLUSIONS

We have characterized the complete Arthrobacter sp. PAMC25564 genome and used comparative analysis to provide insight into the redundancy of its CAZymes for potential cold adaptation. This provides a foundation to understanding how the Arthrobacter strain produces energy in an extreme environment, which is by way of CAZymes, consistent with reports on the use of these specialized enzymes in cold environments. Knowledge of glycogen metabolism and cold adaptation mechanisms in Arthrobacter species may promote in-depth research and subsequent application in low-temperature biotechnology.

Structure of the Mycobacterium smegmatis α-maltose-1-phosphate synthase GlgM

Mycobacterium tuberculosis produces glycogen (also known as α-glucan) to help evade human immunity. This pathogen uses the GlgE pathway to generate glycogen rather than the more well known glycogen synthase GlgA pathway, which is absent in this bacterium. Thus, the building block for this glucose polymer is α-maltose-1-phosphate rather than an NDP-glucose donor. One of the routes to α-maltose-1-phosphate is now known to involve the GlgA homologue GlgM, which uses ADP-glucose as a donor and α-glucose-1-phosphate as an acceptor. To help compare GlgA (a GT5 family member) with GlgM enzymes (GT4 family members), the X-ray crystal structure of GlgM from Mycobacterium smegmatis was solved to 1.9 Å resolution. While the enzymes shared a GT-B fold and several residues responsible for binding the donor substrate, they differed in some secondary-structural details, particularly in the N-terminal domain, which would be expected to be largely responsible for their different acceptor-substrate specificities.

Recent progress in the structure of glycogen serving as a durable energy reserve in bacteria

Glycogen is conventionally considered as a transient energy reserve that can be rapidly synthesized for glucose accumulation and mobilized for ATP production. However, this conception is not completely applicable to prokaryotes due to glycogen structural heterogeneity. A number of studies noticed that glycogen with small average chain length g_c in bacteria has the potential to degrade slowly, which might prolong bacterial environment survival. This phenomenon was previously examined and later formulated as the durable energy storage mechanism hypothesis. Although recent research has been warming to the hypothesis, experimental validation is still missing at current stage. In this review, we summarized recent progress of the hypothesis, provided a supporting mathematical model, and explored the technical pitfalls that shall be avoided in glycogen study.

Mycobacterial OtsA Structures Unveil Substrate Preference Mechanism and Allosteric Regulation by 2-Oxoglutarate and 2-Phosphoglycerate

Mycobacterial infections are a significant source of mortality worldwide, causing millions of deaths annually. Trehalose is a multipurpose disaccharide that plays a fundamental structural role in these organisms as a component of mycolic acids, a molecular hallmark of the cell envelope of mycobacteria. Here, we describe the first mycobacterial OtsA structures. We show mechanisms of substrate preference and show that OtsA is regulated allosterically by 2-oxoglutarate and 2-phosphoglycerate at an interfacial site. These results identify a new allosteric site and provide insight on the regulation of trehalose synthesis through the OtsAB pathway in mycobacteria.

Trehalose is an essential disaccharide for mycobacteria and a key constituent of several cell wall glycolipids with fundamental roles in pathogenesis. Mycobacteria possess two pathways for trehalose biosynthesis. However, only the OtsAB pathway was found to be essential in Mycobacterium tuberculosis, with marked growth and virulence defects of OtsA mutants and strict essentiality of OtsB2. Here, we report the first mycobacterial OtsA structures from Mycobacterium thermoresistibile in both apo and ligand-bound forms. Structural information reveals three key residues in the mechanism of substrate preference that were further confirmed by site-directed mutagenesis. Additionally, we identify 2-oxoglutarate and 2-phosphoglycerate as allosteric regulators of OtsA. The structural analysis in this work strongly contributed to define the mechanisms for feedback inhibition, show different conformational states of the enzyme, and map a new allosteric site.

Structural basis of glycogen metabolism in bacteria

The evolution of metabolic pathways is a major force behind natural selection. In the spotlight of such process lies the structural evolution of the enzymatic machinery responsible for the central energy metabolism. Specifically, glycogen metabolism has emerged to allow organisms to save available environmental surplus of carbon and energy, using dedicated glucose polymers as a storage compartment that can be mobilized at future demand. The origins of such adaptive advantage rely on the acquisition of an enzymatic system for the biosynthesis and degradation of glycogen, along with mechanisms to balance the assembly and disassembly rate of this polysaccharide, in order to store and recover glucose according to cell energy needs. The first step in the classical bacterial glycogen biosynthetic pathway is carried out by the adenosine 5'-diphosphate (ADP)-glucose pyrophosphorylase. This allosteric enzyme synthesizes ADP-glucose and acts as a point of regulation. The second step is carried out by the glycogen synthase, an enzyme that generates linear α-(1→4)-linked glucose chains, whereas the third step catalyzed by the branching enzyme produces α-(1→6)-linked glucan branches in the polymer. Two enzymes facilitate glycogen degradation: glycogen phosphorylase, which functions as an α-(1→4)-depolymerizing enzyme, and the debranching enzyme that catalyzes the removal of α-(1→6)-linked ramifications. In this work, we rationalize the structural basis of glycogen metabolism in bacteria to the light of the current knowledge. We describe and discuss the remarkable progress made in the understanding of the molecular mechanisms of substrate recognition and product release, allosteric regulation and catalysis of all those enzymes.

Structure and Evolution of Glycogen Branching Enzyme N-Termini From Bacteria

In bacteria, glycogen plays important roles in carbon and energy storage. Its structure has recently been linked with bacterial environmental durability. Among the essential genes for bacterial glycogen metabolism, the glgB-encoded branching enzyme GBE plays an essential role in forming α-1,6-glycosidic branching points, and determines the unique branching patterns in glycogen. Previously, evolutionary analysis of a small sets of GBEs based on their N-terminal domain organization revealed that two types of GBEs might exist: (1) Type 1 GBE with both N1 and N2 (also known as CBM48) domains and (2) Type 2 GBE with only the N2 domain. In this study, we initially analyzed N-terminal domains of 169 manually reviewed bacterial GBEs based on hidden Markov models. A previously unreported group of GBEs (Type 3) with around 100 amino acids ahead of the N1 domains was identified. Phylogenetic analysis found clustered patterns of GBE types in certain bacterial phyla, with the shorter, Type 2 GBEs predominantly found in Gram-positive species, while the longer Type 1 GBEs are found in Gram-negative species. Several in vitro studies have linked N1 domain with transfer of short oligosaccharide chains during glycogen formation, which could lead to small and compact glycogen structures. Compact glycogen degrades more slowly and, as a result, may serve as a durable energy reserve, contributing to the enhanced environmental persistence for bacteria. We were therefore interested in classifying GBEs based on their N-terminal domain via large-scale sequence analysis. In addition, we set to understand the evolutionary patterns of different GBEs through phylogenetic analysis at species and sequence levels. Three-dimensional modeling of GBE N-termini was also performed for structural comparisons. A further study of 9,387 GBE sequences identified 147 GBEs that might belong to a possibly novel group of Type 3 GBE, most of which fall into the phylum of Actinobacteria. We also attempted to correlate glycogen average chain length (ACL) with GBE types. However, no significant conclusions were drawn due to limited data availability. In sum, our study systematically investigated bacterial GBEs in terms of domain organizations from evolutionary point of view, which provides guidance for further experimental study of GBE N-terminal functions in glycogen structure and bacterial physiology.

Zwitterionic pyrrolidene-phosphonates: inhibitors of the glycoside hydrolase-like phosphorylase Streptomyces coelicolor GlgEI-V279S

We synthesized and evaluated new zwitterionic inhibitors against glycoside hydrolase-like phosphorylase Streptomyces coelicolor (Sco) GlgEI-V279S which plays a role in α-glucan biosynthesis. Sco GlgEI-V279S serves as a model enzyme for validated anti-tuberculosis (TB) target Mycobacterium tuberculosis (Mtb) GlgE. Pyrrolidine inhibitors 5 and 6 were designed based on transition state considerations and incorporate a phosphonate on the pyrrolidine moiety to expand the interaction network between the inhibitor and the enzyme active site. Compounds 5 and 6 inhibited Sco GlgEI-V279S with K_i = 45 ± 4 μM and 95 ± 16 μM, respectively, and crystal structures of Sco GlgE-V279S-5 and Sco GlgE-V279S-6 were obtained at a 3.2 Å and 2.5 Å resolution, respectively.

Glycan Phosphorylases in Multi-Enzyme Synthetic Processes

Glycoside phosphorylases catalyse the reversible synthesis of glycosidic bonds by glycosylation with concomitant release of inorganic phosphate. The equilibrium position of such reactions can render them of limited synthetic utility, unless coupled with a secondary enzymatic step where the reaction lies heavily in favour of product. This article surveys recent works on the combined use of glycan phosphorylases with other enzymes to achieve synthetically useful processes.

Trehalose-6-Phosphate-Mediated Toxicity Determines Essentiality of OtsB2 in Mycobacterium tuberculosis In Vitro and in Mice

Trehalose biosynthesis is considered an attractive target for the development of antimicrobials against fungal, helminthic and bacterial pathogens including Mycobacterium tuberculosis. The most common biosynthetic route involves trehalose-6-phosphate (T6P) synthase OtsA and T6P phosphatase OtsB that generate trehalose from ADP/UDP-glucose and glucose-6-phosphate. In order to assess the drug target potential of T6P phosphatase, we generated a conditional mutant of M. tuberculosis allowing the regulated gene silencing of the T6P phosphatase gene otsB2. We found that otsB2 is essential for growth of M. tuberculosis in vitro as well as for the acute infection phase in mice following aerosol infection. By contrast, otsB2 is not essential for the chronic infection phase in mice, highlighting the substantial remodelling of trehalose metabolism during infection by M. tuberculosis. Blocking OtsB2 resulted in the accumulation of its substrate T6P, which appears to be toxic, leading to the self-poisoning of cells. Accordingly, blocking T6P production in a ΔotsA mutant abrogated otsB2 essentiality. T6P accumulation elicited a global upregulation of more than 800 genes, which might result from an increase in RNA stability implied by the enhanced neutralization of toxins exhibiting ribonuclease activity. Surprisingly, overlap with the stress response caused by the accumulation of another toxic sugar phosphate molecule, maltose-1-phosphate, was minimal. A genome-wide screen for synthetic lethal interactions with otsA identified numerous genes, revealing additional potential drug targets synergistic with OtsB2 suitable for combination therapies that would minimize the emergence of resistance to OtsB2 inhibitors.

Trehalose biosynthesis is considered an attractive target for the development of new drugs against various microbial pathogens including Mycobacterium tuberculosis. In this human pathogen, two partially redundant pathways mediate trehalose biosynthesis. The OtsA-OtsB2 pathway, which dominates in culture, involves trehalose-6-phosphate (T6P) synthase OtsA and T6P phosphatase OtsB2. While OtsA is dispensable, OtsB2 is strictly essential for growth of M. tuberculosis. Using conditional gene silencing, we here show that essentiality of OtsB2 is linked to accumulation of its substrate T6P, which exhibits direct or indirect toxic effects. Regulated gene expression in vivo revealed that OtsB2 is required to establish an acute infection of M. tuberculosis in a mouse infection model, but is surprisingly fully dispensable during the chronic infection phase. This highlights that trehalose metabolism of M. tuberculosis is substantially remodelled during infection.

The Production and Utilization of GDP-glucose in the Biosynthesis of Trehalose 6-Phosphate by Streptomyces venezuelae

Trehalose-6-phosphate synthase OtsA from streptomycetes is unusual in that it uses GDP-glucose as the donor substrate rather than the more commonly used UDP-glucose. We now confirm that OtsA from Streptomyces venezuelae has such a preference for GDP-glucose and can utilize ADP-glucose to some extent too. A crystal structure of the enzyme shows that it shares twin Rossmann-like domains with the UDP-glucose-specific OtsA from Escherichia coli. However, it is structurally more similar to Streptomyces hygroscopicus VldE, a GDP-valienol-dependent pseudoglycosyltransferase enzyme. Comparison of the donor binding sites reveals that the amino acids associated with the binding of diphosphoribose are almost all identical in these three enzymes. By contrast, the amino acids associated with binding guanine in VldE (Asn, Thr, and Val) are similar in S. venezuelae OtsA (Asp, Ser, and Phe, respectively) but not conserved in E. coli OtsA (His, Leu, and Asp, respectively), providing a rationale for the purine base specificity of S. venezuelae OtsA. To establish which donor is used in vivo, we generated an otsA null mutant in S. venezuelae. The mutant had a cell density-dependent growth phenotype and accumulated galactose 1-phosphate, glucose 1-phosphate, and GDP-glucose when grown on galactose. To determine how the GDP-glucose is generated, we characterized three candidate GDP-glucose pyrophosphorylases. SVEN_3027 is a UDP-glucose pyrophosphorylase, SVEN_3972 is an unusual ITP-mannose pyrophosphorylase, and SVEN_2781 is a pyrophosphorylase that is capable of generating GDP-glucose as well as GDP-mannose. We have therefore established how S. venezuelae can make and utilize GDP-glucose in the biosynthesis of trehalose 6-phosphate.

Structural basis of phosphatidyl-myo-inositol mannosides biosynthesis in mycobacteria

Phosphatidyl-myo-inositol mannosides (PIMs) are glycolipids of unique chemical structure found in the inner and outer membranes of the cell envelope of all Mycobacterium species. The PIM family of glycolipids comprises phosphatidyl-myo-inositol mono-, di-, tri-, tetra-, penta-, and hexamannosides with different degrees of acylation. PIMs are considered not only essential structural components of the cell envelope but also the precursors of lipomannan and lipoarabinomannan, two major lipoglycans implicated in host-pathogen interactions. Since the description of the complete chemical structure of PIMs, major efforts have been committed to defining the molecular bases of its biosynthetic pathway. The structural characterization of the integral membrane phosphatidyl-myo-inositol phosphate synthase (PIPS), and that of three enzymes working at the protein-membrane interface, the phosphatidyl-myo-inositol mannosyltransferases A and B, and the acyltransferase PatA, established the basis of the early steps of the PIM pathway at the molecular level. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

Ligand-bound Structures and Site-directed Mutagenesis Identify the Acceptor and Secondary Binding Sites of Streptomyces coelicolor Maltosyltransferase GlgE

GlgE is a maltosyltransferase involved in α-glucan biosynthesis in bacteria that has been genetically validated as a target for tuberculosis therapies. Crystals of the Mycobacterium tuberculosis enzyme diffract at low resolution so most structural studies have been with the very similar Streptomyces coelicolor GlgE isoform 1. Although the donor binding site for α-maltose 1-phosphate had been previously structurally defined, the acceptor site had not. Using mutagenesis, kinetics, and protein crystallography of the S. coelicolor enzyme, we have now identified the +1 to +6 subsites of the acceptor/product, which overlap with the known cyclodextrin binding site. The sugar residues in the acceptor subsites +1 to +5 are oriented such that they disfavor the binding of malto-oligosaccharides that bear branches at their 6-positions, consistent with the known acceptor chain specificity of GlgE. A secondary binding site remote from the catalytic center was identified that is distinct from one reported for the M. tuberculosis enzyme. This new site is capable of binding a branched α-glucan and is most likely involved in guiding acceptors toward the donor site because its disruption kinetically compromises the ability of GlgE to extend polymeric substrates. However, disruption of this site, which is conserved in the Streptomyces venezuelae GlgE enzyme, did not affect the growth of S. venezuelae or the structure of the polymeric product. The acceptor subsites +1 to +4 in the S. coelicolor enzyme are well conserved in the M. tuberculosis enzyme so their identification could help inform the design of inhibitors with therapeutic potential.

Assembly of α-Glucan by GlgE and GlgB in Mycobacteria and Streptomycetes

Actinomycetes, such as mycobacteria and streptomycetes, synthesize α-glucan with α-1,4 linkages and α-1,6 branching to help evade immune responses and to store carbon. α-Glucan is thought to resemble glycogen except for having shorter constituent linear chains. However, the fine structure of α-glucan and how it can be defined by the maltosyl transferase GlgE and branching enzyme GlgB were not known. Using a combination of enzymolysis and mass spectrometry, we compared the properties of α-glucan isolated from actinomycetes with polymer synthesized in vitro by GlgE and GlgB. We now propose the following assembly mechanism. Polymer synthesis starts with GlgE and its donor substrate, α-maltose 1-phosphate, yielding a linear oligomer with a degree of polymerization (∼16) sufficient for GlgB to introduce a branch. Branching involves strictly intrachain transfer to generate a C chain (the only constituent chain to retain its reducing end), which now bears an A chain (a nonreducing end terminal branch that does not itself bear a branch). GlgE preferentially extends A chains allowing GlgB to act iteratively to generate new A chains emanating from B chains (nonterminal branches that themselves bear a branch). Although extension and branching occur primarily with A chains, the other chain types are sometimes extended and branched such that some B chains (and possibly C chains) bear more than one branch. This occurs less frequently in α-glucans than in classical glycogens. The very similar properties of cytosolic and capsular α-glucans from Mycobacterium tuberculosis imply GlgE and GlgB are sufficient to synthesize them both.

Developmental delay in a Streptomyces venezuelae glgE null mutant is associated with the accumulation of α-maltose 1-phosphate

The GlgE pathway is thought to be responsible for the conversion of trehalose into a glycogen-like α-glucan polymer in bacteria. Trehalose is first converted to maltose, which is phosphorylated by maltose kinase Pep2 to give α-maltose 1-phosphate. This is the donor substrate of the maltosyl transferase GlgE that is known to extend α-1,4-linked maltooligosaccharides, which are thought to be branched with α-1,6 linkages. The genome of Streptomyces venezuelae contains all the genes coding for the GlgE pathway enzymes but none of those of related pathways, including glgC and glgA of the glycogen pathway. This provides an opportunity to study the GlgE pathway in isolation. The genes of the GlgE pathway were upregulated at the onset of sporulation, consistent with the known timing of α-glucan deposition. A constructed ΔglgE null mutant strain was viable but showed a delayed developmental phenotype when grown on maltose, giving less cell mass and delayed sporulation. Pre-spore cells and spores of the mutant were frequently double the length of those of the wild-type, implying impaired cross-wall formation, and spores showed reduced tolerance to stress. The mutant accumulated α-maltose 1-phosphate and maltose but no α-glucan. Therefore, the GlgE pathway is necessary and sufficient for polymer biosynthesis. Growth of the ΔglgE mutant on galactose and that of a Δpep2 mutant on maltose were analysed. In both cases, neither accumulation of α-maltose 1-phosphate/α-glucan nor a developmental delay was observed. Thus, high levels of α-maltose 1-phosphate are responsible for the developmental phenotype of the ΔglgE mutant, rather than the lack of α-glucan.