1/1/08, 12:35

Dissection of the Bradyrhizobium japonicum NifA+r54 regulon, and identification of a ferredoxin gene (fdxN) for symbiotic nitrogen fixation

El presente trabajo aporta importante información sobre la regulación de la simbiosis de Bradyrhizobium japonicum con soja en el proceso de fijación biológica de nitrógeno, identificanco un gen codificante de una ferredoxina intermediaria en el mencionado proceso.

Mol Genet Genomics (2007) 278:255-271 
Felix Hauser - Gabriella Pessi - Markus Friberg - Christoph Weber - Nicola Rusca - Andrea Lindemann - Hans- Martin Fischer - Hauke Hennecke

Hierarchically organized regulatory proteins form a complex network for expression control of symbiotic and accessory genes in the nitrogen-fixing soybean symbiont Bradyrhizobium japonicum. A genome-wide survey of regulatory interactions was made possible with the design of a custom-made gene chip. Here, we report the first use of the microarray in a comprehensive and complete characterization of the B. japonicum NifA+r54 regulon which forms an important node in the entire network. Comparative transcript profiles of anaerobically grown wild-type, nifA, and rpoN1/2 mutant cells were complemented with a position-specific frequency matrix-based search for NifA- and r54- binding sites plus a simple operon definition. One of the newly identified NifA+r54-dependent genes, fdxN, encodes a ferredoxin required for efficient symbiotic nitrogen fixation, which makes it a candidate for being a direct electron donor to nitrogenase. The fdxN gene has an unconventional, albeit functional r54 promoter with the dinucleotide GA instead of the consensus GC motif at position -12. A GC-containing mutant promoter and the atypical GA-containing promoter of the wild type were disparately activated. Expression analyses were also carried out with two other NifA+r54 targets (ectC; ahpC). Incidentally, the tiling-like design of the microarray has helped to arrive at completely revised annotations of the ectC- and ahpC-upstream DNA regions, which are now compatible with promoter locations. Taken together, the approaches used here led to a substantial expansion of the NifA+r54 regulon size, culminating in a total of 65 genes for nitrogen fixation and diverse other processes.