HOPANOID LIPIDS COMPOSE THE FRANKIA VESICLE ENVELOPE, PRESUMPTIVE BARRIER OF OXYGEN DIFFUSION TO NITROGENASE

Biological nitrogen fixation in aerobic organisms requires a mechanism for excluding oxygen from the site of nitrogenase activity. Oxygen ex clusion in Frankia spp., members of an actinomycetal genus that forms nitrogen-fixing root-nodule symbioses in a wide range of woody Angiosp erms, is accomplished within specialized structures termed vesicles, w here nitrogen fixation is localized. The lipidic vesicle envelope is a pparently a functional analogue of the cyanobacterial heterocyst envel ope, forming an external gas-diffusion barrier around the nitrogen-fix ing cells. We report here that purified vesicle envelopes consist prim arily of two hopanoid lipids, rather than of glycolipids, as is the ca se in cyanobacteria. One envelope hopanoid, bacteriohopanetetrol pheny lacetate monoester, is vesicle-specific. The Frankia vesicle envelope thus represents a layer specific to the locus of nitrogen fixation tha t is biosynthetically uniquely derived.