John S. Kim

Relationships between activities of xylanases and xylan structures.

Structures of five water-soluble xylans have been determined. Four purified xylanase enzymes have been studied for the hydrolysis of the xylans. Different xylanases have different activities against various xylan structures. The key factors that influence the rate of xylan hydrolysis are chain length and degree of substitution. Two family 11 xylanases, Orpinomyces pc2 xylanase and Trichoderma longibrachiatum xylanase, can rapidly hydrolyze xylans that have a chain length greater than 8 xylose residues, and their hydrolytic rates are not sensitive to substituents on the xylan backbone. A family 11 xylanase from Aureobasidium pullulans is most effective on xylans that have a long chain (greater than 19 xylose residues), and also is effective against substituent groups. Although Thermatoga maritima xylanase is also more active on a long xylan chain (greater than 19 xylose residues), its hydrolytic rate is greatly reduced by substituents on xylan backbones.

The presence of a novel type of surface polysaccharide in Rhizobium meliloti requires a new fatty acid synthase-like gene cluster involved in symbiotic nodule development

Bacterial exopolysaccharide (EPS) and lipopolysaccharide (LPS) molecules have been shown to play important roles in plant‐bacterium interactions. Here we have demonstrated that the fix‐23 loci, which compensate for exo mutations during symbiotic nodule development, are involved in the production of a novel polysaccharide that is rich in 3‐deoxy‐D manno‐2‐octulosonic acid (Kdo) but is not the classical LPS. This molecule is likely to be a surface antigen since antiserum to whole Rhizobium meliloti cells reacts strongly with it, and since mutations in fix‐23 result in an inability to produce this polysaccharide and to bind bacteriophage 16‐3. It is likely that this Kdo‐rich polysaccharide is analogous to certain Escherichia coli K‐antigens which are anchored to the membrane via a phospholipid moiety. DNA sequence analysis of one gene cluster of this region revealed that the predicted protein products of six genes exhibit a high degree of homology and similar organization to those of the rat fatty acid synthase multifunctional enzyme domains.

Role of the K-antigen subgroup of capsular polysaccharides in the early recognition process between Rhizobium meliloti and alfalfa leaves

We used a model system to investigate the induction of plant genes by bacterial surface compounds. We have infiltrated alfalfa leaves with wild-type Rhizobium meliloti strain Rm41 and mutant derivatives, which are deficient in the production of exopolysaccharides (EPS), capsular K-polysaccharides (KPS or K-Antigens), or both. We have shown that wild-type R. meliloti was able to induce transcript accumulation of genes encoding enzymes of the iso-flavonoid biosynthetic pathway: chalcone synthase (CHS), chalcone reductase (CHR), and isoflavone reductase (IFR). Kinetics of these transcript accumulations were biphasic, occurring 0.75 to 1.5 h and 6 to 30 h after treatment. The exoB derivative of Rm41 (strain AK631), which is unable to produce either EPS I or EPS II, was still able to induce very rapid (45-min posttreatment) transcript accumulation of CHS and CHR genes. These results suggested that EPS were not involved in this induction. We then tested three R. meliloti fix-23 mutants (strains PP699, PP711, an...