COEXISTENCE OF 2 STRUCTURALLY SIMILAR BUT FUNCTIONALLY DIFFERENT P-IIPROTEINS IN AZOSPIRILLUM-BRASILENSE

The coexistence of two different P-II proteins in Azospirillum brasile nse was established by comparing proteins synthesized by the wild-type strain and two null mutants of the characterized glnB gene (encoding P,) adjacent to glnA, Strains were grown under conditions of nitrogen limitation or nitrogen excess, The proteins were analyzed by sodium do decyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) or isoelec tric focusing gel electrophoresis and revealed either by [P-32]phospha te or [H-3]uracil labeling or by cross-reaction with an anti-ii. brasi lense P-II-antiserum, After SDS-PAGE, a single band of 12.5 kDa reveal ed by the antiserum in all conditions tested was resolved by isoelectr ic focusing electrophoresis into two bands in the wild-type strain, on e of which was absent in the glnB null mutant strains, The second P-II protein, named P-Z, was uridylylated under conditions of nitrogen lim itation. The amino acid sequence deduced from the nucleotide sequence of the corresponding structural gene, called glnZ, is very similar to that of P-II. Null mutants in glnB were impaired in regulation of nitr ogen fixation and in their swarming properties but not in glutamine sy nthetase adenylylation, No glnZ mutant is yet available, but it is cle ar that P-II and P-Z are not functionally equivalent, since glnB null mutant strains exhibit phenotypic characters, The two proteins are pro bably involved in different regulatory steps of the nitrogen metabolis m in A. brasilense.