(7,12). Here, we have investigated whether the central domain of DctD, like the central domain of NifA, is transcriptionally competent in the absence of the amino-and carboxy-terminal domains.We made an initial deletion of a portion of the DctD amino-terminal domain which removed the first 81 amino acid residues of a total of 121 amino acids in the aminoterminal domain (pJS5). This was accomplished by making a transcriptional fusion of the dctD gene to the lacZ promoter of pUC18 at a Sall site in the middle of the DctD aminoterminal domain. An AUG codon located 16 bp downstream from the fusion junction is the probable translational start for this truncated DctD protein. A second transcriptional fusion of the lacZ promoter to the entire dctBD operon (pJS7) was constructed by cloning a 2.7-kb BamHI-SalI fragment carrying dctB and the 5' end of dctD into the Sall site of pJS5, thereby reconstructing the dctD gene. A translational fusion of the detA promoter to the Escherichia coli lacZ gene was

No abstract

The rpoN (ntrA) gene (encoding sigma 54) of Azospirillum brasilense Sp7 was isolated by using conserved rpoN primers and the polymerase chain reaction, and its nucleotide sequence was determined. The deduced amino acid sequence of the RpoN protein was found to share a high degree of homology with other members of the sigma 54 family. Two additional open reading frames were found in the Azospirillum brasilense rpoN region, with significant similarity to equivalent regions surrounding the rpoN locus in other bacteria. An rpoN mutant of Azospirillum brasilense Sp7 was constructed by gene replacement and found to be defective in nitrogen fixation, nitrate assimilation, and ammonium uptake. Lack of ammonium uptake was also found in previously isolated Azospirillum brasilense ntrB and ntrC mutants, further supporting the role of the ntr system in this process. In addition, the rpoN mutant was found to be nonmotile, suggesting a role of RpoN in Azospirillum brasilense flagellar biosynthesis.

No abstract

Azorhizobium caulinodans ORS571 was found to excrete moderate amounts of a fluorescent pigment into the culture medium in response to dissolved oxygen tensions below 1.0 kPa. The pigment was identified as coproporphyrin, on the basis of its optical and fluorescence spectra. FixLJ and fixK mutant derivatives of ORS571 were found to excrete 25-fold higher amounts of coproporphyrin under micro-aerobic conditions than the wild type strain. These observations suggest that A. caulinodans switches from an aerobic to an anaerobic coproporphyrinogen oxidase when the dissolved oxygen tension falls below 1.0 kPa and that the fixLJ and fixK genes are involved in the regulation of expression of the anaerobic coproporphyrinogen oxidase.