REGULATION OF NIF GENE-EXPRESSION IN ENTEROBACTER-AGGLOMERANS - NUCLEOTIDE-SEQUENCE OF THE NIFLA OPERON AND INFLUENCE OF TEMPERATURE AND AMMONIUM ON ITS TRANSCRIPTION

The nucleotide sequence of a plasmid-borne 3.9 kb XhoI-SmaI fragment c omprising the 3'-region of the nifM gene, the nifL and nifA genes and the 5'-region of nifB gene of Enterobacter agglomerans was determined. The genes were identified by their homology to the corresponding nif genes of Klebsiella pneumoniae. A typical sigma(54)-dependent promoter and a consensus NtrC-binding motif were identified upstream of nifL. The predicted amino acid sequence of NifL showed close similarities to NifL of K. pneumoniae and Azotobacter vinelandii. However, no histidi ne residue was found to correspond to histidine-304 of A. vinelandii N ifL, which had been proposed to be required for the repressor activity of NifL. The NifA sequence with a putative DNA binding motif (Q(x(3)) A(x(3)) G(x(5))I) and an ATP binding site in the C-terminal and centr al domains, respectively, resembles that of other known NifA proteins. The function of the nifL and nifA genes was demonstrated in vivo usin g a binary plasmid system by their ability to activate a nifH promoter -lacZ fusion at different temperatures and concentrations of NH4+. Max imal promoter activity occurred at 25 degrees C, and it appears that t he sensitivity of NifA to elevated temperatures is independent of NifL . The expression of nifL inhibited promoter activity in the presence o f NifA when the initial NH4+ concentration in the medium exceeded 4 mM .