Characterization of <i>Frankia casuarinae</i> Mutants Defective in Vesicle Envelope Development

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 N₂. 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 N₂ due to the inactivation of nitrogenase caused by oxygen penetration into the vesicles.