RESUMEN
In our endeavor to improve the nitrogen fixation efficiency of a soil diazotroph that would be unaffected by synthetic nitrogenous fertilizers, we have deleted a part of the negative regulatory gene nifL and constitutively expressed the positive regulatory gene nifA in the chromosome of Azotobacter chroococcum CBD15, a strain isolated from the local field soil. No antibiotic resistance gene or other foreign gene was present in the chromosome of the engineered strain. Wheat seeds inoculated with this engineered strain, which we have named Azotobacter chroococcum HKD15, were tested for 3 years in pots and 1 year in the field. The yield of wheat was enhanced by â¼60% due to inoculation of seeds by A. chroococcum HKD15 in the absence of any urea application. Ammonium only marginally affected acetylene reduction by the engineered Azotobacter strain. When urea was also applied, the same wheat yield could be sustained by using seeds inoculated with A. chroococcum HKD15 and using â¼85 kg less urea (â¼40 kg less nitrogen) than the usual â¼257 kg urea (â¼120 kg nitrogen) per hectare. Wheat plants arising from the seeds inoculated with the engineered Azotobacter strain exhibited far superior overall performance, had much higher dry weight and nitrogen content, and assimilated molecular 15N much better. A nitrogen balance experiment also revealed much higher total nitrogen content. Indole-3-acetic acid (IAA) production by the wild type and that by the engineered strain were about the same. Inoculation of the wheat seeds with A. chroococcum HKD15 did not adversely affect the microbial population in the field rhizosphere soil.IMPORTANCE Application of synthetic nitrogenous fertilizers is a standard agricultural practice to augment crop yield. Plants, however, utilize only a fraction of the applied fertilizers, while the unutilized fertilizers cause grave environmental problems. Wild-type soil diazotrophic microorganisms cannot replace synthetic nitrogenous fertilizers, as these reduce atmospheric nitrogen very inefficiently and almost none at all in the presence of added nitrogenous fertilizers. If the nitrogen-fixing ability of soil diazotrophs could be improved and sustained even in the presence of synthetic nitrogenous fertilizers, then a mixture of the bacteria and a reduced quantity of chemical nitrogenous fertilizers could be employed to obtain the same grain yield but at a much-reduced environmental cost. The engineered Azotobacter strain that we have reported here has considerably enhanced nitrogen fixation and excretion abilities and can replace â¼85 kg of urea per hectare but sustain the same wheat yield, if the seeds are inoculated with it before sowing.
RESUMEN
Nitrogen fixation in Azotobacter vinelandii is regulated by the nifLA operon. NifA activates the transcription of nif genes, while NifL antagonizes the transcriptional activator NifA in response to fixed nitrogen and molecular oxygen levels. However, transcriptional regulation of the nifLA operon of A. vinelandii itself is not fully understood. Using the S1 nuclease assay, we mapped the transcription start site of the nifLA operon, showing it to be similar to the sigma54-dependent promoters. We also identified a positive cis-acting regulatory element (+134 to +790) of the nifLA operon within the coding region of the nifL gene of A. vinelandii. Deletion of this element results in complete loss of promoter activity. Several protein factors bind to this region, and the specific binding sites have been mapped by DNase I foot printing. Two of these sites, namely dR1 (+134 to +204) and dR2 (+745 to +765), are involved in regulating the nifLA promoter activity. The absence of NtrC-like binding sites in the upstream region of the nifLA operon in A. vinelandii makes the identification of these downstream elements a highly significant finding. The interaction of the promoter with the proteins binding to the dR2 region spanning +745 to +765 appears to be dependent on the face of the helix as introduction of 4 bases just before this region completely disrupts promoter activity. Thus, the positive regulatory element present within the BglII-BglII fragment may play, in part; an important role in nifLA regulation in A. vinelandii.
