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Biosynthesis and function of extracellular glycans in cyanobacteria.

Kehr JC, Dittmann E - Life (Basel) (2015)

Bottom Line: The complex carbohydrates act as barriers against different types of stress and play a role in intra- as well as inter-species interactions.We discuss similarities with well-studied enterobacterial systems and highlight the unique features of cyanobacteria.We pay special attention to colony formation and EPS biosynthesis in the bloom-forming cyanobacterium, Microcystis aeruginosa.

View Article: PubMed Central - PubMed

Affiliation: University of Potsdam, Institute for Biochemistry and Biology, Department of Microbiology, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany. jckehr@uni-potsdam.de.

ABSTRACT
The cell surface of cyanobacteria is covered with glycans that confer versatility and adaptability to a multitude of environmental factors. The complex carbohydrates act as barriers against different types of stress and play a role in intra- as well as inter-species interactions. In this review, we summarize the current knowledge of the chemical composition, biosynthesis and biological function of exo- and lipo-polysaccharides from cyanobacteria and give an overview of sugar-binding lectins characterized from cyanobacteria. We discuss similarities with well-studied enterobacterial systems and highlight the unique features of cyanobacteria. We pay special attention to colony formation and EPS biosynthesis in the bloom-forming cyanobacterium, Microcystis aeruginosa.

No MeSH data available.


Models of three distinct EPS biosynthesis routes in E. coli. In the Wzx/Wzy-dependent system, repeat units are synthesized at the cytoplasmatic site of the inner membrane and translocated into the periplasm by the flippase, Wzx. Wzy assembles the repeat units to the final polysaccharide. In the Wzm/Wzt system, the complete polysaccharide is synthesized at the inner membrane and exported by the ABC transporter Wzm/Wzt. In the synthase-dependent system, chain elongation is directly linked to transport, but the details are unknown.
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life-05-00164-f002: Models of three distinct EPS biosynthesis routes in E. coli. In the Wzx/Wzy-dependent system, repeat units are synthesized at the cytoplasmatic site of the inner membrane and translocated into the periplasm by the flippase, Wzx. Wzy assembles the repeat units to the final polysaccharide. In the Wzm/Wzt system, the complete polysaccharide is synthesized at the inner membrane and exported by the ABC transporter Wzm/Wzt. In the synthase-dependent system, chain elongation is directly linked to transport, but the details are unknown.

Mentions: Several biosynthetic routes in Gram-negative bacteria were elucidated, and the corresponding genes were identified [20,21,23,24,25,26]. Commonly, the genes involved in exopolysaccharide biosynthesis are clustered, and the nomenclature is consistent among different species, while in most cyanobacteria, the genes are clustered in smaller units or even orphaned and dispersed over the whole chromosome. Additionally, automated genome annotation led to misannotations and an inconsistent naming. Therefore, the detailed description of glycan biosynthesis below will follow the general scheme for Gram-negative bacteria and highlight differences described in cyanobacteria. Since only a few cyanobacterial pathways have been analyzed in detail, it is not clear if the mechanisms apply to all cyanobacterial genera. Capsular polysaccharides in E.coli are classified into four groups, where Groups 1 and 4 and Groups 2 and 3 share similar biosynthesis routes [25]. The proteins that facilitate initiation and transport of the glycopolymer are conserved and present in all members of each group, while the presence of serotype-specific enzymes providing monosaccharide building blocks and glycosyltransferases linking these to the growing polysaccharide chain determines the specific composition of the glycan [25]. Group 1 and 4 polysaccharides are assembled by the Wzx/Wzy systems, and Group 2 and 3 glycans depend on the Wzt/Wzm (ABC transporter-dependent) system (Figure 2). In both cases, biosynthesis is initiated at the cytosolic face of the inner membrane at an integral membrane glycosyltransferase by the transfer of the first building block to a lipid carrier. The following steps differ significantly between both systems. In the Wzx/Wzy system, individual repeating units are assembled and transferred to the periplasmic side of the inner membrane by the flippase Wzx and are subsequently linked by the Wzy protein to the growing polysaccharide chain. This process further requires the integral membrane protein Wzc and the associated phosphatase Wzb (Figure 2). Finally, the nascent polymer is translocated through the outer membrane by the channel Wza. In contrast, in the ABC transporter-dependent Wzt/Wzm (kpsT/kpsM)-dependent system, polysaccharides are completely assembled at the inner cytoplasmic membrane and secreted by the ABC transporter, Wzt/Wzm. Later, a synthase-dependent pathway was discovered, in which the export is directly linked to polysaccharide synthesis [27]. This type of polysaccharide biosynthesis was shown to be involved in the synthesis of poly-β-1,6-N-acetyl-d-glucosamine encoded by the pgaABCD gene cluster [28] and cellulose encoded by the bcsABZC gene cluster [29] in E. coli.


Biosynthesis and function of extracellular glycans in cyanobacteria.

Kehr JC, Dittmann E - Life (Basel) (2015)

Models of three distinct EPS biosynthesis routes in E. coli. In the Wzx/Wzy-dependent system, repeat units are synthesized at the cytoplasmatic site of the inner membrane and translocated into the periplasm by the flippase, Wzx. Wzy assembles the repeat units to the final polysaccharide. In the Wzm/Wzt system, the complete polysaccharide is synthesized at the inner membrane and exported by the ABC transporter Wzm/Wzt. In the synthase-dependent system, chain elongation is directly linked to transport, but the details are unknown.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4390846&req=5

life-05-00164-f002: Models of three distinct EPS biosynthesis routes in E. coli. In the Wzx/Wzy-dependent system, repeat units are synthesized at the cytoplasmatic site of the inner membrane and translocated into the periplasm by the flippase, Wzx. Wzy assembles the repeat units to the final polysaccharide. In the Wzm/Wzt system, the complete polysaccharide is synthesized at the inner membrane and exported by the ABC transporter Wzm/Wzt. In the synthase-dependent system, chain elongation is directly linked to transport, but the details are unknown.
Mentions: Several biosynthetic routes in Gram-negative bacteria were elucidated, and the corresponding genes were identified [20,21,23,24,25,26]. Commonly, the genes involved in exopolysaccharide biosynthesis are clustered, and the nomenclature is consistent among different species, while in most cyanobacteria, the genes are clustered in smaller units or even orphaned and dispersed over the whole chromosome. Additionally, automated genome annotation led to misannotations and an inconsistent naming. Therefore, the detailed description of glycan biosynthesis below will follow the general scheme for Gram-negative bacteria and highlight differences described in cyanobacteria. Since only a few cyanobacterial pathways have been analyzed in detail, it is not clear if the mechanisms apply to all cyanobacterial genera. Capsular polysaccharides in E.coli are classified into four groups, where Groups 1 and 4 and Groups 2 and 3 share similar biosynthesis routes [25]. The proteins that facilitate initiation and transport of the glycopolymer are conserved and present in all members of each group, while the presence of serotype-specific enzymes providing monosaccharide building blocks and glycosyltransferases linking these to the growing polysaccharide chain determines the specific composition of the glycan [25]. Group 1 and 4 polysaccharides are assembled by the Wzx/Wzy systems, and Group 2 and 3 glycans depend on the Wzt/Wzm (ABC transporter-dependent) system (Figure 2). In both cases, biosynthesis is initiated at the cytosolic face of the inner membrane at an integral membrane glycosyltransferase by the transfer of the first building block to a lipid carrier. The following steps differ significantly between both systems. In the Wzx/Wzy system, individual repeating units are assembled and transferred to the periplasmic side of the inner membrane by the flippase Wzx and are subsequently linked by the Wzy protein to the growing polysaccharide chain. This process further requires the integral membrane protein Wzc and the associated phosphatase Wzb (Figure 2). Finally, the nascent polymer is translocated through the outer membrane by the channel Wza. In contrast, in the ABC transporter-dependent Wzt/Wzm (kpsT/kpsM)-dependent system, polysaccharides are completely assembled at the inner cytoplasmic membrane and secreted by the ABC transporter, Wzt/Wzm. Later, a synthase-dependent pathway was discovered, in which the export is directly linked to polysaccharide synthesis [27]. This type of polysaccharide biosynthesis was shown to be involved in the synthesis of poly-β-1,6-N-acetyl-d-glucosamine encoded by the pgaABCD gene cluster [28] and cellulose encoded by the bcsABZC gene cluster [29] in E. coli.

Bottom Line: The complex carbohydrates act as barriers against different types of stress and play a role in intra- as well as inter-species interactions.We discuss similarities with well-studied enterobacterial systems and highlight the unique features of cyanobacteria.We pay special attention to colony formation and EPS biosynthesis in the bloom-forming cyanobacterium, Microcystis aeruginosa.

View Article: PubMed Central - PubMed

Affiliation: University of Potsdam, Institute for Biochemistry and Biology, Department of Microbiology, Karl-Liebknecht-Str. 24/25, 14476 Potsdam-Golm, Germany. jckehr@uni-potsdam.de.

ABSTRACT
The cell surface of cyanobacteria is covered with glycans that confer versatility and adaptability to a multitude of environmental factors. The complex carbohydrates act as barriers against different types of stress and play a role in intra- as well as inter-species interactions. In this review, we summarize the current knowledge of the chemical composition, biosynthesis and biological function of exo- and lipo-polysaccharides from cyanobacteria and give an overview of sugar-binding lectins characterized from cyanobacteria. We discuss similarities with well-studied enterobacterial systems and highlight the unique features of cyanobacteria. We pay special attention to colony formation and EPS biosynthesis in the bloom-forming cyanobacterium, Microcystis aeruginosa.

No MeSH data available.