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Section 9: Regulation of N2 Fixation and Metabolism
CHAIR'S COMMENTS: RECENT PROGRESS IN THE REGULATION OF NITROGEN FIXATION AND NITROGEN ASSIMILATION GENES
Microbiologic et Environnement, URA CNRS 2172, Institut Pasteur, 25 rue du Dr. Roux, 75724 Paris Cedex 15, France
As new diazotrophic species are discovered (e.g. Moulin et al. 2001), there is continuous progress in the number of species for which we gather advanced molecular genetics of the «¡/and fix clusters (e.g. Lee et al. 2000; Desnoues et al. this volume). In addition, genomics projects for an increasing number of bacteria bring about new perspectives in our knowledge of the degree of conservation and evolution of nitrogen fixation and nitrogen assimilation genes.
Although several regulatory genes are common to most diazotrophs, in particular within the proteobacteria, the regulatory cascades differ from one species to the other. The case of Bradyrhizobium japonicum was reviewed by Hans-Martin Fischer. This bacterium contains two oxygen-responsive regulatory cascades: the FixLJ-K2 system controls the functions in anaerobic and microaerobic metabolism, whereas the RegSR-NifA system controls genes essential for symbiotic nitrogen fixation. The use of competitive DNA-RNA hybridization techniques identified new NifA regulated genes that appeared not to be essential for the symbiosis (Nienaber et al. 2000). Further analysis of the RegSR two-component system was presented including the identification of a RegR DNA-binding site and the description of RegSR homologs present in other bacteria. Cyanobacteria, which are phylogenetically distant from proteobacteria, possess a completely different network. Enrique Flores described the properties of NtcA, a transcriptional regulator protein from the cAMP receptor protein family, that binds DNA sequences in the promoter region of cyanobacterial genes governing nitrogen sources utilization and heterocyst differentiation (Herrero et al. 2000). A model of heterocyst development was then discussed. A substantial amount of information on the cascades in R. etli, S. meliloti, A. vinelandii, Azospirillum, Herbaspirillum and different photosynthetic bacteria was also reported during the Poster Presentation Session.
2. NifM is a Peptidylproline cis-trans Isomerase (PPIase)
Occurrence of a nifM (or nifM-like) gene seems to be limited to members of the gamma subgroup of proteobacteria, including non-nitrogen-fixing species, such as Escherichia coli and Pseudomonas aeruginosa. An analysis of the role of the NifM protein of A. vinelandii was reported by Nara Gavini. NifM was found to display PPIase activity. An interaction between NifM and the nitrogenase Fe protein was detected using the yeast two-hybrid technique.
The involvement of PII and parologs in the regulatory cascades remain of major interest. PII proteins are small trimeric proteins playing an overall role in ammonia-sensing mechanisms (Arcondeguy et al. 2001). Most nitrogen fixers studied so far carry two copies of glnB-like genes, namely glnB and glnK. A single copy, glnK, was found in A. vinelandii, whereas three copies, glnB, glnK and glnY are present in Azoarcus (Martin et al. 2000) and in Rhodospirillum rubrum (Zhang et al. this volume). In general, bacterial genes encoding ammonium transporters (amtB) are located downtream of glnK genes. AmtB is a membrane-anchored protein that belongs to a family of transporters conserved in bacteria, fungi, plants and animals. The purification and properties of the
AmtB protein in E. coli was reported by Mike Merrick, who also showed how GlnK could associate with AmtB in the membrane and proposed a model of regulation of ammonium transport by PII.
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