Cloning of the ADPglucose pyrophosphorylase (glgC) and glycogen synthase (glgA) structural genes from Salmonella typhimurium LT2

A straightforward assay for measuring glycogen levels and RpoS

Cellular glycogen levels reflect the activity of RpoS, an important stress-inducible bacterial sigma factor known to regulate several stress-resistance related genes, such as katE, encoding hydroperoxidase II (HPII), and the glg genes, encoding glycogen synthesis enzymes, in Escherichia coli. In this study, a straightforward assay for measuring glycogen levels and RpoS activity was developed combining the ease and simplicity of qualitative approaches. The assay reagent was a 2% iodine solution (2% iodine/1M NaOH), and the basic principle of this assay is the iodine-glycogen reaction, which produces a reddish brown color that can be measured using a spectrophotometer. A calibration plot using a known amount of glycogen yielded the best linear fit over a range of 10-300μg/assay (R²=0.994). The applicability of the assay for measuring the glycogen level of various samples was assessed using a wild type (WT) E. coli K-12 strain, glycogen- and RpoS-deficient isogenic mutants, and clinical bacterial isolates with or without RpoS activity; the assay generated reproducible results. Additionally, the assay was successfully applied for measuring glycogen levels in human cells. In conclusion, we developed a straightforward and cost-effective assay for measuring glycogen levels, which can be applied for measuring RpoS activity.

Glycogen: Biosynthesis and Regulation

Glycogen accumulation occurs in Escherichia coli and Salmonella enterica serovar Typhimurium as well as in many other bacteria. Glycogen will be formed when there is an excess of carbon under conditions in which growth is limited because of the lack of a growth nutrient, e.g., a nitrogen source. This review describes the enzymatic reactions involved in glycogen synthesis and the allosteric regulation of the first enzyme, ADP-glucose pyrophosphorylase. The properties of the enzymes involved in glycogen synthesis, ADP-glucose pyrophosphorylase, glycogen synthase, and branching enzyme are also characterized. The data describing the genetic regulation of the glycogen synthesis are also presented. An alternate pathway for glycogen synthesis in mycobacteria is also described.

Toll-like receptor 2 activation by Chlamydia trachomatis is plasmid dependent, and plasmid-responsive chromosomal loci are coordinately regulated in response to glucose limitation by C. trachomatis but not by C. muridarum

We previously demonstrated that plasmid-deficient Chlamydia muridarum retains the ability to infect the murine genital tract but does not elicit oviduct pathology because it fails to activate Toll-like receptor 2 (TLR2). We derived a plasmid-cured derivative of the human genital isolate Chlamydia trachomatis D/UW-3/Cx, strain CTD153, which also fails to activate TLR2, indicating this virulence phenotype is associated with plasmid loss in both C. trachomatis and C. muridarum. As observed with plasmid-deficient C. muridarum, CTD153 displayed impaired accumulation of glycogen within inclusions. Transcriptional profiling of the plasmid-deficient strains by using custom microarrays identified a conserved group of chromosomal loci, the expression of which was similarly controlled in plasmid-deficient C. muridarum strains CM972 and CM3.1 and plasmid-deficient C. trachomatis CTD153. However, although expression of glycogen synthase, encoded by glgA, was greatly reduced in CTD153, it was unaltered in plasmid-deficient C. muridarum strains. Thus, additional plasmid-associated factors are required for glycogen accumulation by this chlamydial species. Furthermore, in C. trachomatis, glgA and other plasmid-responsive chromosomal loci (PRCLs) were transcriptionally responsive to glucose limitation, indicating that additional regulatory elements may be involved in the coordinated expression of these candidate virulence effectors. Glucose-limited C. trachomatis displayed reduced TLR2 stimulation in an in vitro assay. During human chlamydial infection, glucose limitation may decrease chlamydial virulence through its effects on plasmid-responsive chromosomal genes.

Glycogen: Biosynthesis and Regulation

The accumulation of glycogen occurs in Escherichia coli and Salmonella enterica serovar Typhimurium as well as in many other bacteria. Glycogen will be formed when there is an excess of carbon under conditions in which growth is limited due to the lack of a growth nutrient, e.g., a nitrogen source. The structural genes of the glycogen biosynthetic enzymes of E. coli and S. serovar Typhimurium have been cloned previously, and that has provided insights in the genetic regulation of glycogen synthesis. An important aspect of the regulation of glycogen synthesis is the allosteric regulation of the ADP-Glc PPase. The current information, views, and concepts regarding the regulation of enzyme activity and the expression of the glycogen biosynthetic enzymes are presented in this review. The recent information on the amino acid residues critical for the activity of both glycogen synthase and branching enzyme (BE) is also presented. The residue involved in catalysis in the E. coli ADP-Glc PPase was determined by comparing a predicted structure of the enzyme with the known three-dimensional structures of sugar-nucleotide PPase domains. The molecular cloning of the E. coliglg K-12 structural genes greatly facilitated the subsequent study of the genetic regulation of bacterial glycogen biosynthesis. Results from studies of glycogen excess E. coli B mutants SG3 and AC70R1, which exhibit enhanced levels of the enzymes in the glycogen synthesis pathway (i.e., they are derepressed mutants), suggested that glycogen synthesis is under negative genetic regulation.

Glycogen production by different Salmonella enterica serotypes: contribution of functional glgC to virulence, intestinal colonization and environmental survival

In enteric bacteria, the contribution of endogenous energy sources to survival both inside and outside the host is poorly understood. The contribution of glycogen production to the virulence, colonization and environmental survival of different Salmonella enterica serotypes was assessed. Of 19 serotypes (339 strains) tested for glycogen production, 17 (256 strains) were positive. The avian-specific serovars S. Gallinarum (62 strains) and S. Pullorum (21 strains) did not produce glycogen. The sequence of glgC in three S. Gallinarum strains tested revealed an identical deletion of 11 consecutive bases, which was not present in S. Pullorum, and a CCC insertion after position 597. Transduction of S. Gallinarum and S. Pullorum to a glycogen-positive phenotype did not change the ability to colonize the intestine or affect virulence in the chicken. Mortality rates in chickens following oral infection with a S. Typhimurium glycogen mutant (glgC : : km) were not significantly reduced, although colonization of the intestine was reduced over the first 4 weeks of the trial. Growth and yield of the glgC : : km mutant were comparable to the parent. The glgC mutant survived less well in faeces and in water at 4 degrees C when the strain was grown in LB broth containing 0.5 % glucose, and in saline it died off more rapidly after 7 days. The data suggest that glycogen has a complex but comparatively minor role in virulence and colonization, but a more significant role in survival.

A Streptococcus mutans mutant that synthesizes elevated levels of intracellular polysaccharide is hypercariogenic in vivo

We used the streptococcal transposon, Tn916 to identify and isolate mutants of Streptococcus mutans with altered intracellular polysaccharide (IPS) accumulation. We report on the isolation and characterization of S. mutans SMS202, a transposon mutant which accumulated the glycogen-like IPS in excess of wild-type levels. Southern blot analysis confirmed a single Tn916 insertion into the SMS202 chromosome. Moreover, quantitative ultrastructural analysis revealed significantly increased concentrations of IPS in SMS202 relative to those of the wild-type progenitor strain, UA130. The activities of ADPglucose pyrophosphorylase (GlgC) and glycogen synthase (GlgA), enzymes required for the biosynthesis of bacterial IPS, were also elevated in the IPS excess mutant. Furthermore, SMS202 was significantly more cariogenic on the molar surfaces of germ-free rats than the wild type (P < 0.01), thus confirming a central role for IPS in S. mutants-induced caries formation. We propose that the increased cariogenic potential of SMS202 is due to constitutive expression of genes which encode glycogen biosynthesis in this oral pathogen. The coordinate expression of GlgC and GlgA along with the results of ongoing nucleotide sequence analysis and Northern hybridization experiments support an operon-like arrangement for the glg genes of this oral pathogen.

A simple method for cloning genes involved in glucan biosynthesis: isolation of structural and regulatory genes for glycogen synthesis in Escherichia coli

A simple and widely applicable method for cloning genes involved in glucan biosynthesis is described. An Escherichia coli genomic library was prepared in the low-copy plasmid, pLG339, and E. coli transformants from this library were screened by staining with iodine vapor. Colonies that stained darker than the control were isolated and characterized. The three classes of clones that were identified included: (i) plasmids encoding E. coli glycogen biosynthetic (glg) structural genes, (ii) clones that resulted in elevated glycogen levels, but did not encode glg structural genes or enhance the level of the first enzyme of the pathway, ADPglucose pyrophosphorylase (AGPP), and (iii) clones that enhanced the level of AGPP, but did not encode this enzyme. Two clones from the latter class also enhanced glgC'-'lacZ-encoded beta-galactosidase activity, and may encode factors that regulate the expression of glg structural genes. It should be possible to readily clone glycogen biosynthetic genes from other bacterial species via this method. The method could be made specific for a desired glg gene by using a recipient strain that is defective in the gene of interest.

Cloning and characterization of the asd gene of Salmonella typhimurium: use in stable maintenance of recombinant plasmids in Salmonella vaccine strains

The asd mutants of Salmonella typhimurium have an obligate requirement for diaminopimelic acid (DAP) and will undergo lysis in environments deprived of DAP. This has allowed the development of a balanced-lethal system for the expression of heterologous antigens in vaccine strains using vectors containing the wild-type asd gene from Streptococcus mutans and asd mutant Salmonella hosts [Nakayama et al., Biotechnology 6 (1988) 693-697]. We have cloned the asd gene from S. typhimurium, characterized the gene product and used this gene to construct Asd+ expression cloning vectors. In addition we have constructed an asd cassette and a transposon derived from Tn5 that allow the rapid modification of other vectors for use with delta asd vaccine strains of S. typhimurium adding versatility to the Asd+ vector/delta asd host system of plasmid maintenance.

Cloning and expression of the branching enzyme gene (glgB) from the cyanobacterium Synechococcus sp. PCC7942 in Escherichia coli

Using the glgB gene from Escherichia coli as a hybridization probe, the gene encoding the branching enzyme of the cyanobacterium Synechococcus sp. PCC7942 has been identified on a 3.9-kb PstI fragment which was cloned into plasmid pUC9. Two types of plasmids have been isolated. Plasmid pKVN1 was expressing the Synechococcus sp. gene as was shown by complementation of the glgB mutation of E. coli KV832. Plasmid pKVN2, which carried the same insert in the opposite orientation was unable to complement E. coli KV832, indicating that the promoter of the cloned gene was either absent or was not recognized in E. coli. Determination of branching activity in extracts of Synechococcus sp. and E. coli KV832[pKVN1] showed that the enzyme was optimally active at approximately 35 degrees C. No significant activity was present at temperatures higher than 55 degrees C, reflecting the mesophilic nature of the cloned enzyme. In a cell-free coupled transcription-translation system the cloned gene specified two proteins of 84 kDa and 72 kDa, respectively, which are probably translated independently from the same gene by initiation at two different start codons.

Molecular cloning and sequencing of the glycogen phosphorylase gene from Escherichia coli

The glgP gene, which codes for glycogen phosphorylase, was cloned from a genomic library of Escherichia coli. The nucleotide sequence of the glgP gene contained a single open reading frame encoding a protein consisting of 790 amino acid residues. The glgP gene product, a polypeptide of Mr 87,000, was confirmed by SDS-polyacrylamide gel electrophoresis. The deduced amino acid sequence showed that homology between glgP of E. coli and rabbit glgP, human glgP, potato glgP, and E. coli malP was 48.6, 48.6, 42.3, and 46.1%, respectively. Within this homologous region, the active site, glycogen storage site, and pyridoxal-5'-phosphate binding site are well conserved. The enzyme activity of glycogen phosphorylase increased after introduction on a multicopy of the glgP gene.

Biosynthesis of bacterial glycogen: primary structure of Salmonella typhimurium ADPglucose synthetase as deduced from the nucleotide sequence of the glgC gene

The nucleotide sequence of a 1.4-kilobase-pair fragment containing the Salmonella typhimurium LT2 glgC gene coding for ADPglucose synthetase was determined. The glgC structural gene contains 1,293 base pairs, having a coding capacity of 431 amino acids. The amino acid sequence deduced from the nucleotide sequence shows that the molecular weight of ADPglucose synthetase is 45,580. Previous results of the total amino acid composition analysis and amino acid sequencing (M. Lehmann and J. Preiss, J. Bacteriol. 143:120-127, 1980) of the first 27 amino acids from the N terminus agree with that deduced from nucleotide sequencing data. Comparison of the Escherichia coli K-12 and S. typhimurium LT2 ADPglucose synthetase shows that there is 80% homology in their nucleotide sequence and 90% homology in their deduced amino acid sequence. Moreover, the amino acid residues of the putative allosteric sites for the physiological activator fructose bisphosphate (amino acid residue 39) and inhibitor AMP (amino acid residue 114) are identical between the two enzymes. There is also extensive homology in the putative ADPglucose binding site. In both E. coli K-12 and S. typhimurium LT2, the first base of the translational start ATG of glgA overlaps with the third base TAA stop codon of the glgC gene.