Symbioses: Plant, Animal, and Microbe Interactions
Hartmann, Linda , Barnum, S R .
Nif genes in symbiotically-capable Nostoc: More than one form?
Although 80% of the atmosphere is composed of dinitrogen, living organisms rely on fixed forms of nitrogen. The ability to fix nitrogen is widely, but sporadically, dispersed among prokaryotes. Cyanobacteria are one of the largest groups of nitrogen fixing organisms, with some strains forming symbiotic associations with plants and other organisms. Some filamentous cyanobacteria fix atmospheric nitrogen in specialized, terminally differentiated heterocysts where the nitrogen-fixing enzyme, nitrogenase, is protected from oxygen. In many cyanobacteria, heterocyst patterning is highly regulated resulting in spacing every 10-12 cells along the filament. However, when a plant-cyanobacteria symbiosis is established, the frequency of heterocysts may increase ten-fold with a corresponding decrease in phostsosynthetic activity. In this study, we sequenced the nifK genes from Nostoc strains to determine the phylogenetic relationships of nifK genes between host-specific and non-specific strains. While the phylogeny of nifK did not support clusters of symbiotically-capable strains, we found two Nostoc strains (PCC 9303 and 8307 that form symbiotic associations with Azolla) that encode a second form of nifK (nifK2). These genes are very similar to the one in Anabaena variabilis (ATTCC 29413) that is expressed only in vegetative cells under microaerobic conditions. Because Nostoc cyanobionts are enclosed in mucous-filled cavities in the plant host, oxygen levels may be low and nitrogen fixation may occur in both heterocysts and vegetative cells in some strains.
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1 - Miami University, Botany, Pearson Hall, Oxford, Ohio, 45056, United States
Presentation Type: Poster:Posters for Topics
Location: Ball Room & Party Room/SUB
Date: Monday, July 28th, 2008
Time: 12:30 PM