Regulation of nif gene expression and nitrogen metabolism in Azospirillum

The Azospirillum genus comprises nitrogen-fixing bacteria which have been isolated from the roots of numerous grasses, including cereals, sugar cane and forage grasses. Among the five species known, Azospirillum brasilense is the best documented in terms of genetics and regulation of nitrogen fixation and ammonia assimilation. Several key regulatory and structural genes have been characterized, including nifA, ntrBC, glnA, glnB and rpoN. Regulation of their expression has been studied at the free-living state and in association with wheat. A model of regulation is reported and the challenge of engineering nitrogen-fixing associations is also discussed.