SummaryWe observed that the maize pathogenic fungus Ustilago maydis grew in nitrogen (N)-free media at a rate similar to that observed in media containing ammonium nitrate, suggesting that it was able to fix atmospheric N 2 . Because only prokaryotic organisms have the capacity to reduce N 2 , we entertained the possibility that U. maydis was associated with an intracellular bacterium.The presence of nitrogenase in the fungus was analyzed by acetylene reduction, and capacity to fix N 2 by use of 15 N 2 . Presence of an intracellular N 2 -fixing bacterium was analyzed by PCR amplification of bacterial 16S rRNA and nifH genes, and by microscopic observations. Nitrogenase activity and 15 N incorporation into the cells proved that U. maydis fixed N 2 .Light and electron microscopy, and fluorescence in situ hybridization (FISH) experiments revealed the presence of intracellular bacteria related to Bacillus pumilus, as evidenced by sequencing of the PCR-amplified fragments. These observations reveal for the first time the existence of an endosymbiotic N 2 -fixing association involving a fungus and a bacterium.
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