A novel exopolysaccharide can function in place of the calcofluor-binding exopolysaccharide in nodulation of alfalfa by Rhizobium meliloti.
We have found that R. meliloti strain Rm1021, which is known to synthesize a Calcofluor-binding exopolysaccharide (EPS I), also has a cryptic capacity to synthesize a second exopolysaccharide (EPS II). Structural analysis of EPS II has shown that it differs in many respects from EPS I. Genetic analysis indicates that EPS II synthesis requires the products of at least seven loci on the second symbiotic megaplasmid of R. meliloti, and is induced by a mutation, expR101, which causes increased transcription of these genes. Synthesis of EPS II suppresses the symbiotic defects of EPS I-deficient strains on Medicago sativa (alfalfa), but not on four other plants that are normally hosts for Rm1021. These observations suggest that structural features of bacterial exopolysaccharides are involved in the determination of host range. The implications of these results for models of exopolysaccharide function, such as serving as signals to the plant or shielding the bacteria from plant defense responses, are discussed.
Glazebrook
J
J
Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139.
Walker
G C
GC
D001557:Benzenesulfonates / Q000378:metabolism; D002240:Carbohydrate Sequence; D004252:DNA Mutational Analysis; D004251:DNA Transposable Elements; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D009682:Magnetic Resonance Spectroscopy; D000455:Medicago sativa / Q000382:microbiology*; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation; D011135:Polysaccharides, Bacterial / Q000502:physiology*; D015183:Restriction Mapping; D012231:Rhizobium / Q000235:genetics / Q000502:physiology*; D013559:Symbiosis
D001557:Benzenesulfonates; D004251:DNA Transposable Elements; D011135:Polysaccharides, Bacterial; C007061:C.I. Fluorescent Brightening Agent 28