Azospirillum: Genetics of Nitrogen Fixation and Interaction with Plants

Bacteria of the genus Azospirillum are free-living diazotrophs that were isolated from the rhizosphere and from the roots of grasses. Genetic analysis of nitrogen fixation was essentially initiated in A. brasilense Sp7, where genetic tools and mutants are available. A DNA region covering 25 kb and containing the nitrogenase structural genes (nifHDK), nifE and another nif cluster has been cloned. In addition, the structural gene for glutamine synthetase, which might be involved in nif regulation, was cloned and sequenced. To identify bacterial genes involved in the root colonization process, DNA-DNA hybridization was performed with Rhizobium nodulation (nod and hsn) genes. Homology was detected in both cases and clones containing DNA homologous to hsn genes were isolated. Azospirillum contains large plasmids. Preliminary experiments suggest that the hsn homologous region is located on the 90 MDa plasmid of strain Sp7.