Degradation of plant cell wall polysaccharides by Saccharophagus degradans

Saccharophagus degradans is an aerobic, Gram negative marine bacterium, isolated from decaying Spartina alterniflora in the Chesapeake Bay watershed. S. degradans can degrade and metabolize numerous complex polysaccharides, including the major components of the plant cell wall, cellulose, xylan and beta-glucan. Genomic analyses reveal that S. degradans has 77 genes coding for enzymes that are predicted to participate in the degradation of plant cell wall polysaccharides. These include complete, functional, multienzyme systems for the depolymerization of cellulose, xylan, arabinan, beta-1,3-glucans, beta-1,4-glucans, and mannan. Most of the cellulases are modular, some of which contain novel combinations of catalytic and/or substrate binding modules. In addition to its well-predicted plant wall degrading systems, S. degradans encodes 19 proteins which contain a carbohydrate binding module, but lack an identifiable catalytic domain and 12 glycanases for which function cannot be predicted by sequence analysis. Many of the plant wall degrading enzymes contain lipoprotein signature sequences, indicating that they are likely attached to the cell surface, thereby maintaining their reactions near the cell and preventing loss of enzyme or product to diffusion or competition. S. degradans is capable of using crystalline cellulose and intact plant matter as sole carbon and energy sources. Cellulose induces catalytic activity against all major plant cell wall polymers, suggesting a complex mechanism for coordinating the regulation of these multienzyme systems. In addition to its abundant carbohydrases, S. degradans encodes seven proteins with predicted molecular weights over 250,000 Daltons, one of which, CabA at 1,500,000 Daltons, is the largest known bacterial protein to date. These proteins contain calcium-binding repeat sequences suggesting a role in cell-to-surface adhesion or protein-to-protein interaction, perhaps as a means of surface-enzyme attachment. These studies establish S. degradans as the first marine bacterium with a complete and functional cellulase system with the further ability to degrade plants in monoculture.