Categories of exopolysaccharide biosynthesis pathways#

exoPS biosynthesis can be divided into four general pathways, these are the Synthase-dependent, Sucrase-dependent, Wzx/Wzy-dependent and ABC-transporter-dependent pathways. Below is a brief description of each.

Synthase-dependent pathways#

The synthase-dependent bioynthesis pathways include the Bcs operons (cellulose production), the alginate operon, the pel operon (both the Gram-positive and Gram-negative operons) and poly-N-acetyl-glucosamine (PNAG, both the ica and pga operons). In synthase-dependent pathways, polymerisation and secretion into the periplasm/extracellular space is facilitated by a single glycosyltransferase protein (the synthase), which is embedded in the cytoplasmic membrane. In some systems, polymerisation is regulated by a c-di-GMP-binding regulator protein which forms a protein complex with the synthase. In Gram-negative systems the synthase-dependent pathways often include a scaffolding protein which guides the growing polysaccharide to a porin in the outer membrane. Since synthase-dependent systems only utilise a single protein for polymerisation, most exopolysaccharides produced this way are homopolymers. The gene synteny of synthase-dependent BGCs is generally well conserved across taxa.

Sucrase-dependent pathways#

Sucrase-dependent exoPS biosynthesis depends on a single sucrase protein anchored to the cell wall. They are outliers among the other pathways since their "BGCs" only consist of a single sucrase gene. Two types of polysaccharide can be produced by these sucrases, depending on their activity. Glucansucrases transfer glucose from sucrose, creating alpha-glucans, while fructansucrases transfer glucose from sucrose, creating fructans.

Wzx/Wzy-dependent pathways#

The Wzx/Wzy-dependent pathway is the most wide-spread type exoPS biosynthesis pathway in bacteria. Many capsular polysaccharides are created through this pathway, aiding pathogens in host evasion. Plant-associated use the Wzx/Wzy-dependent pathway for EPS assembly in root nodules (e.g. succinoglycan from Sinorhizobium meliloti), and plant pathogens use exoPS from this pathway as virulence factors (e.g. stewartan from Erwinia or xanthan from Xanthomonas). exoPS from the Wzx/Wzy-dependent pathways also play a role in biofilm formation, Psl from Pseudomonas aeruginosa and colanic acid from E. coli being the most well-known examples. In addition to these roles, the Wzx/Wzy-dependent pathway (as well as the ABC-transporter-dependent pathway) is responsible for the synthesis of the O-antigen component of the lipolysaccharides which are an integral part of the outer membranes of all Gram-negative bacteria.

exoPS biosynthesis through the Wzx/Wzy-dependent pathway is initiated by the addition of sugar monomers to a membrane-bound undecaprenyl diphosphate anchor. This is facilitated by a series of cytoplasmic glycosyltransferases and results in an oligosaccharide, called a repeating unit, which is translocated to the periplasm by the flippase Wzx. Here, the repeating unit is attached to the growing polysaccharide by the Wzy polymerase. In collaboration with the polysaccharide co-polymerase (PCP protein), the polysaccharide is guided towards an outer membrane porin, OPX. In the case of lipoppolysaccharides, the finished polysaccharide is not secreted through OPX, but is instead transported to proteins of the lpt pathway where it is combined with lipid A to form a finished lipopolysaccharide.

The exoPS produced by the Wzx/Wzy-dependent pathway have an incredibly diverse composition, with up to five different sugars in their repeating units. This is reflected by their BGCs which have infamously diverse gene compositions and syntenies which are known to change drastically even between species strains.

ABC-transporter-dependent pathways#

The ABC transporter-dependent pathway, like the Wzx/Wzy-dependent pathway, is intracellular and is initiated by the assembly of oligosaccharides by GTs at an inner membrane-bound linker. Unlike in the Wzx/Wzy-dependent pathway, the entire polysaccharide chain is assembled on the linker, and the chain can be either a hetero- or homopolymer. This polymer is exported and translocated across the inner membrane and into the extracellular space by a protein complex consisting of an ABC-transporter, PCP and OPX. As in the Wzx/Wzy-dependent system, the OPX protein is omitted and the polysaccharide is sent to the lpt machinery in the case of lipoplysaccharide synthesis.

Like the Wzx/Wzy-dependent pathways, the ABC-transporter-dependent pathways are notoriuosly diverse in their gene composition and syntenies. The ABC-transporter-dependent pathways are responsible for the synthesis of the O-antigen component of the lipolysaccharides, which are an integral part of the outer membranes of all Gram-negative bacteria, as well as capsular exopolysaccharides.