No abstract

Biological nitrogen fixation in aerobic organisms requires a mechanism for excluding oxygen from the site of nitrogenase activity. Oxygen exclusion in Frankia spp., members of an actinomycetal genus that forms nitrogen-fixing root-nodule symbioses in a wide range of woody Angiosperms, is accomplished within specialized structures termed vesicles, where nitrogen fixation is localized. The lipidic vesicle envelope is apparently a functional analogue of the cyanobacterial heterocyst envelope, forming an external gas-diffusion barrier around the nitrogen-fixing cells. We report here that purified vesicle envelopes consist primarily of two hopanoid lipids, rather than of glycolipids, as is the case in cyanobacteria. One envelope hopanoid, bacteriohopanetetrol phenylacetate monoester, is vesicle-specific. The Frankic vesicle envelope thus represents a layer specific to the locus of nitrogen fixation that is biosynthetically uniquely derived.

Exopolysaccharides (EPS) of the soybean pathogen Pseudomonas syringae pv. glycinea were isolated from culture ifitrates and infected soybean leaves. Levan (a polyfructan with a C-2 --C-6 backbone and C-2 -+ C-1 branching) or acetylated alginate (a linear polyuronide of C-1-+ C-4-linked mannuronic and guluronic acids) was isolated from culture filtrates when bacterial strains were grown in a semisynthetic medium containing sucrose or glucose, respectively, as the primary carbon source. Acetylated alginate was the only EPS isolated from soybean [Glycine max (L.) Merr.] leaves inoculated with compatible (disease-inducing) strains of P. syringae pv. glycinea. The acetyl content of the P. syringae pv. glycinea alginates varied from 3 to 14%, and the amount of guluronic acid varied from less than 1 to 20%. The P. syringae pv. glycinea alginates from in vitro batch cultures were of lower molecular weight and polydispersity thakm those from in planta cultures, and both were of lower molecular weights than alginates produced by Pseudomonas aeruginosa.