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). Structrual 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.
Bibliographical noteFunding Information:
We wish to thank Jason Reed and T. Lynne Reuber for communication of unpublished results and critical reading of the manuscript, Gour-Rong Her and Vernon Reinhold for communication of unpublished results, Shoshona Klein for the gifts of TnSTp and the TnSMob chromosomal mapping strains, Myron Williams for the gift of pEX2, Mary Honma for the gift of seeds, Gary Barsomian for help with DNA cloning techniques, lgor Khait and David Ruben for help with proton NMR spectroscopy, and Patricia Reilly for help with microscopy. J. Glaze-brook was supported by a National Science Foundation predoctoral fellowship.