Frankia vesicles provide inducible and absolute oxygen protection for nitrogenase.

When Frankia HFPCcI3 was grown in culture at oxygen O(2) levels ranging from 2 to 70 kilopascals O(2), under nitrogen fixing conditions, nitrogenase activity adapted to ambient pO(2) and showed a marked optimum close to growth pO(2). Vesicles were thin walled at low pO(2) and very thick walled at high pO(2). Freeze fracture transmission electron microscopy confirmed that Frankia produces vesicles with outer walls thickened by multiple lipid-like monolayers, in proportion to ambient pO(2).