Structural features of the vesicle of Frankia sp. CpI1 in culture

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Abstract
The filamentous bacterium Frankia sp. CpI1 of the Actinomycetales, responsible for symbiotic nitrogen fixation in the nodules of certain woody dicots, also fixes dinitrogen when grown independently of the host in a nitrogen-free synthetic nutrient medium under aerobic conditions. In structural studies of Frankia grown in culture it has been shown that the bacterial filaments form vesicles, enlarged terminal endings in which the enzyme nitrogenase is formed. Microscopic examination of cultures shows that the vesicles possess a specialized envelope consisting of a number of thin layers or laminae which in polarized light show birefringence and in freeze-etch electron microscopy are resolved as multiple (12–15) laminae approximately 35–40 Å (1 Å = 0.1 nm) in thickness. Comparisons are made between the structure of the vesicle envelope in cultured Frankia and the strikingly similar innermost laminated layer in the dinitrogen-fixing heterocysts of the cyanobacterium Anabaena. Comparable protective functions in limiting oxygen to the dinitrogen-fixing sites are suggested for these similar structures in two quite unrelated microorganisms.