Abstract
Frankia, a nitrogen-fixing actinobacterium, forms a unique multicellular structure known as a vesicle that is dedicated to nitrogen fixation. The vesicle is surrounded by a thick hopanoid lipid envelope that acts as a barrier against oxygen penetration, preventing nitrogenase inactivation. Five mutants produced a similar number of vesicles to the wild type; however, they failed to fix N(2). The thickness of vesicle envelopes was reduced in all five mutants, and the oxygen concentration increased inside the vesicles of four mutants. Therefore, these mutants were unable to fix N(2) due to the inactivation of nitrogenase caused by oxygen penetration into the vesicles.