Potential roles for the glnB and ntr YX genes in Azospirillum brasilense

Three Azospirillum brasilense mutants constitutive for nitrogen fixation (N if(C)) in the presence of NH4+ and deficient in nitrate-dependent growth we re used as tools to define the roles of the glnB and ntr YX genes in this o rganism. Mutant HM14 was complemented for nitrate-dependent growth and NH4 regulation of nitrogenase by plasmid pL46 which contains the ntr YX genes of A. brasilense. Mutant HM26 was restored for NH4+ regulation and nitrate- dependent growth by plasmid pJCI, carrying the A. brasilense g1nB gene expr essed from a constitutive promoter. Mutant HM053, on the other hand, was no t complemented for NH4+ regulation of nitrogenase and nitrate-dependent gro wth by both plasmids pJCI and pL46. The levels and control of glutamine syn thetase activity of all mutants were not affected by both plasmids pL46 (nt r YA) and pJCI (glnB). These results support the characterization of strain s HM14 as an ntr YX mutant and strain HM26 as a glnB mutant and the involve ment of ntrYX and glnB in the regulation of the general nitrogen metabolism in A. brasilense. (C) 2001 Federation of European Microbiological Societie s. Published by Elsevier Science B.V.. All rights reserved.