STRUCTURAL ORGANIZATION AND REGULATION OF THE NIF GENES OF HERBASPIRILLUM-SEROPEDICAE

The promoter regions of nifA and nifH genes of Herbaspirillom seropedi cae have been isolated and investigated, and the nifA promoter has bee n characterized in detail. Both regions contain a -24/-12 type promote r and NifA-binding sites; the nifA promoter also has an NtrC-binding s ite. The nifA expression is activated by NtrC and repressed by ammoniu m ions but not by oxygen. When the NtrC-binding site was deleted nifA expression was NifA-dependent. The native NifA protein of H. seropedic ae was unable to activate nifH expression in Escherichia coli whereas a truncated protein, lacking the N-terminal domain was able to activat e nifH expression in the absence of oxygen and presence of iron, indic ating that the N-terminal domain regulates NifA activity. The truncate d protein also activates nifH in Azospirillum brasilense in the presen ce or absence of ammonium ions. This suggests that the N-terminal doma in senses the nitrogen status of the cell. (C) 1997 Elsevier Science L td.