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.