Sphingomonas strains have
extracellular polysaccharide (e.g., gellan, diutan) that is firmly attached to the
cell surface. This attachment may limit
polysaccharide production by impairing uptake of nutrients into the
cell or due to limited sites for
polysaccharide biosynthesis on the
cell surface. Two genes for polysaccharide
biosynthesis, designated gelM and gelN in gellan-producing strains and dpsM and dpsN in diutan-producing strains, have been inactivated by deletion mutations and shown to produce polysaccharide that is not firmly attached to the cell surface, i.e., slime form. Another
gene for polysaccharide
biosynthesis, designated gelI in gellan producing strains, was inactivated by
insertion mutation and also shown to produce the slime
phenotype. The
homologous gene dpsi in the diutan producing strain should also be involved in the attachment of the polysaccharide to the cell surface. The slime characteristic was demonstrated by the ability of the cells to be centrifuged and the lack of cell clumping as seen under the
microscope or in diluted suspensions. The diutan slime mutants had somewhat increased productivity and the recovered diutan product had significantly improved
rheology. Gellan slime mutants had lower broth
viscosity which facilitates mixing during
fermentation; however, the recovered gellan product had lower
gel strength than the gellan produced from a capsular strain. A deletion in a
gene gelR, which encodes a
protein with homology to surface proteins and outer membrane proteins and weak homology to proteins with
polysaccharide degradation activity, was shown to restore higher
gel strength to the slime form of gellan, and to produce gellan of higher
gel strength than that of the capsular gellan producing strains.