Bottom Line:
Osmoregulated periplasmic glucans (OPGs) are oligosaccharides found in the periplasm of many Gram-negative bacteria. In E. coli, OPG are substituted by phosphoglycerol and phosphoethanolamine derived from membrane phospholipids and by succinyl residues. Both genes show structural and functional similarities without sequence similarity. 

ABSTRACTOsmoregulated periplasmic glucans (OPGs) are oligosaccharides found in the periplasm of many Gram-negative bacteria.  Glucose is the sole constitutive sugar and this backbone may be substituted by various kinds of molecules depending on the species.  In E. coli, OPG are substituted by phosphoglycerol and phosphoethanolamine derived from membrane phospholipids and by succinyl residues.  In this study, we describe the isolation of the opgE gene encoding the phosphoethanolamine transferase by a screen previously used for the isolation of the opgB gene encoding the phosphoglycerol transferase.  Both genes show structural and functional similarities without sequence similarity. 

Mentions:
Osmoregulated periplasmic glucans (OPGs), formerly membrane derived oligosaccharides are oligosaccharides accumulated in the envelope of many Gram-negative bacteria in media of low osmolarity.  They belong to the common virulence factors found in phyto- and zoo pathogen of many Gram-negative bacterial species.  In addition, OPGs control motility and secretion of exopolysaccharides in several species [7–9].  Glucose is the sole constitutive sugar and this glucosidic backbone may be substituted by various substituents depending on the species [10].  In Escherichia coli, OPG backbone is synthesized by the opgGH products (Figure 1) [11].  Total loss of OPGs reduced motility and increased exopolysaccharides synthesis and they are phenotypes associated with inactivation of opgG or opgH genes.  OpgH is a transmembrane glucosyl-transferase catalyzing, with acyl carrier protein (ACP) as a cofactor and UDP-glucose as a substrate, the synthesis of a backbone constituted of linear β-1,2-linked glucose units.  OpgG is a periplasmic glucosyl transferase branching glucose units on this linear backbone by β-1,6-linkages [10].  This backbone is highly substituted by phosphoglycerol residues, and to a less extent by phosphoethanolamine residues, derived from membrane phosphatidylglycerol and phosphatidylethanolamine, respectively, and by succinyl residues (Figure 1) [10].  Two phosphoglycerol transferases catalyze the substitution by phosphoglycerol and are encoded by the same opgB gene [12].  The phosphoglycerol transferase I is anchored to the inner membrane but a large periplasmic catalytic domain transfers phosphoglycerol residues from phosphatidylglycerols to OPG molecules.  The phosphoglycerol transferase II is a periplasmic soluble enzyme resulting from the liberation of the periplasmic catalytic domain of the former one and catalyzes the transfer of phosphoglycerol residues from one OPG molecule to another.  Both phosphoglycerol transferases can transfer phosphoglycerol to artificial β-glucoside acceptors such as arbutin leading to enhanced turnover of phosphatidylglycerol.  This was the basis of the selection of the mdoB mutants.  In a dgk strain grown in media of low osmolarity containing arbutin, accumulation of diacylglycerol occurred in membrane to a toxic level and cell growth slowed abruptly.  In a double mutant dgk opgB, diacylglycerol accumulation decreased and growth resumed [13].  The succinyl-transferase is encoded by the opgC gene and is an intrinsic transmembrane protein catalyzing the transfer of succinyl residues probably from succinyl-CoA to OPG molecules.  In an opgB strain, anionic character of OPGs is only provided by succinyl residues since phosphoethanolamine is a neutral substituent.  This was the basis of the isolation of the opgC mutant which was isolated by screening an opgB strain using the severe difference of migration within a thin layer chromatography between neutral and anionic OPGs [11]. 

Mentions:
Osmoregulated periplasmic glucans (OPGs), formerly membrane derived oligosaccharides are oligosaccharides accumulated in the envelope of many Gram-negative bacteria in media of low osmolarity.  They belong to the common virulence factors found in phyto- and zoo pathogen of many Gram-negative bacterial species.  In addition, OPGs control motility and secretion of exopolysaccharides in several species [7–9].  Glucose is the sole constitutive sugar and this glucosidic backbone may be substituted by various substituents depending on the species [10].  In Escherichia coli, OPG backbone is synthesized by the opgGH products (Figure 1) [11].  Total loss of OPGs reduced motility and increased exopolysaccharides synthesis and they are phenotypes associated with inactivation of opgG or opgH genes.  OpgH is a transmembrane glucosyl-transferase catalyzing, with acyl carrier protein (ACP) as a cofactor and UDP-glucose as a substrate, the synthesis of a backbone constituted of linear β-1,2-linked glucose units.  OpgG is a periplasmic glucosyl transferase branching glucose units on this linear backbone by β-1,6-linkages [10].  This backbone is highly substituted by phosphoglycerol residues, and to a less extent by phosphoethanolamine residues, derived from membrane phosphatidylglycerol and phosphatidylethanolamine, respectively, and by succinyl residues (Figure 1) [10].  Two phosphoglycerol transferases catalyze the substitution by phosphoglycerol and are encoded by the same opgB gene [12].  The phosphoglycerol transferase I is anchored to the inner membrane but a large periplasmic catalytic domain transfers phosphoglycerol residues from phosphatidylglycerols to OPG molecules.  The phosphoglycerol transferase II is a periplasmic soluble enzyme resulting from the liberation of the periplasmic catalytic domain of the former one and catalyzes the transfer of phosphoglycerol residues from one OPG molecule to another.  Both phosphoglycerol transferases can transfer phosphoglycerol to artificial β-glucoside acceptors such as arbutin leading to enhanced turnover of phosphatidylglycerol.  This was the basis of the selection of the mdoB mutants.  In a dgk strain grown in media of low osmolarity containing arbutin, accumulation of diacylglycerol occurred in membrane to a toxic level and cell growth slowed abruptly.  In a double mutant dgk opgB, diacylglycerol accumulation decreased and growth resumed [13].  The succinyl-transferase is encoded by the opgC gene and is an intrinsic transmembrane protein catalyzing the transfer of succinyl residues probably from succinyl-CoA to OPG molecules.  In an opgB strain, anionic character of OPGs is only provided by succinyl residues since phosphoethanolamine is a neutral substituent.  This was the basis of the isolation of the opgC mutant which was isolated by screening an opgB strain using the severe difference of migration within a thin layer chromatography between neutral and anionic OPGs [11]. 

Bottom Line:
Osmoregulated periplasmic glucans (OPGs) are oligosaccharides found in the periplasm of many Gram-negative bacteria. In E. coli, OPG are substituted by phosphoglycerol and phosphoethanolamine derived from membrane phospholipids and by succinyl residues. Both genes show structural and functional similarities without sequence similarity. 

ABSTRACTOsmoregulated periplasmic glucans (OPGs) are oligosaccharides found in the periplasm of many Gram-negative bacteria.  Glucose is the sole constitutive sugar and this backbone may be substituted by various kinds of molecules depending on the species.  In E. coli, OPG are substituted by phosphoglycerol and phosphoethanolamine derived from membrane phospholipids and by succinyl residues.  In this study, we describe the isolation of the opgE gene encoding the phosphoethanolamine transferase by a screen previously used for the isolation of the opgB gene encoding the phosphoglycerol transferase.  Both genes show structural and functional similarities without sequence similarity.