| 摘要 |
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.
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