In order to understand a process as complex as nitrogen fixation and to be in a position to manipulate it for the benefit of mankind, researchers are now working at the frontiers of science in many different areas: protein structure and function; catalytic mechanisms; electron transfer processes; regulatory circuits and environmental sensing; metabolic integration; chemical communication between organisms; differentiation; genome structure and function; microbial ecology; plant physiology; plant molecular biology; and agronomy. This volume represents a testimony to the advances in nitrogen fixation research that have been made and the contribution of these efforts to the solution of many other varied scientific problems. Limiting steps for future advances are analyzed and new horizons in nitrogen fixation research are proposed.

Zusammenfassung
Proceedings of the 9th International Congress on Nitrogen Fixation, Cancun, Mexico, December 6-12, 1992

Inhalt
Introductory remarks at the overview session.- Nitrogenases: Distribution, composition, structure and function.- Rhizobium and legume nodulation: A molecular dialogue.- Plant function in nodulation and nitrogen fixation in legumes.- Organization and regulation of nitrogen fixation genes.- The role of respiration in symbiotic nitrogen fixation.- Evolution since Knoxville: Were nitrogen-fixing organisms wise to inhabit land plants?.- An integrated, interdisciplinary approach to understanding the chemistry of nitrogenases.- Structures and functions of the nitrogenase proteins.- The crystal structure of the nitrogenase MoFe protein from Clostridium pasteurianum.- Kinetics and mechanism of nitrogenase a role for 'P' centres in dinitrogen reduction.- Nif gene products and their roles in nitrogen fixation.- Intermolecular electron transfer and substrate reduction properties of MoFe proteins altered by site-specific amino acid substitution.- Use of Fe protein altered at specific amino acid positions to probe its function in nitrogenase catalysis.- Metal regulation of nitrogenases and molybdenum transport in Azotobactervinelandii.- Regulatory role of molecular chaperones in nitrogenase biogenesis.- Nod factors from Rhizobium fredii USDA257.- The sulfate group on the reducing end protects nod signals of R. meliloti against hydrolysis by Medicaqo chitinases.- The function of the rhizobial nodABC and nodFEL operons in the biosynthesis of lipo-oligosaccharides.- Chemical signaling of Rhizobium nodulating bean.- Bradyrhizobium japonicum strains produce multiple 2-0-methylfucose-containing lipo-oligosaccharide nodulation signals.- Nod genes and nod factors of Rhizobium species NGR234.- Nod factor production by Azorhizobium caulinodans strain ORS571.- Signals released fromalfalfa regulate microbial activities in the rhizosphere.- Exopolysaccharides and Rhizobium meliloti-alfalfa interactions.- The role of Rhizobium meliloti surface polysaccharides in nodule development.- Rhizobium lequminosarum CE3 lipopolysaccharide structure required in symbiosis with bean.- Pea nod- mutants and nod factor perception.- Biogenesis of peribacteroid membrane (PMB) forming a subcellular compartment essential for symbiotic nitrogen fixation.- Characterization of GmENOD40, a gene expressed in soybean nodule primordia.- Expression of the Medicaqo truncatula ENOD12 gene in response to R. meliloti nod factors and during spontaneous nodulation in transgenic alfalfa.- Molecular interactions during nodule development.- Nodule morphogenesis in the absence of Rhizobium.- Ethylene and nodulation.- Differential gene expression in the development of actinorhizal root nodules.- A cyanobacterial-plant endosymbiosis: Aspects of its establishment.- Potential targets and pathways for Rhizobium's mitogenic stimulus.- Molecular analysis of components responsible for protection of Azotobacter nitrogenase from oxygen damage.- The regulation of oxygen and its role in regulating nodule metabolism.- In vivo and in vitro oxygen control of nitrogen fixation gene expression in Rhizobium meliloti.- Oxygen sensing by the Rhizobium meliloti two-component regulatory system, FixLJ.- Two oxygen-responsive regulatory cascades control nitrogen fixation genes in Bradyrhizobium japonicum.- Heterocyst differentiation and nitrogen fixation in the cyanobacterium Anabaena.- Regulation of nifA synthesis and activity in Azospirillum brasilense Sp7.- The nifL gene of Azotobactervinelandii: Novel features of sequence, expression and mutant phenotypes.- A maltose-binding protein-nifA (MBP-NIFA)fusion activates transcription from the Klebsiella pneumoniae nifH promoter in a purified system: Stimulation by IHF.- Carbon and nitrogen metabolism in Rhizobium.- Succinate metabolism in Rhizobium meliloti.- Diversion of carbon to polysaccharide synthesis by Bradyrhizobium japonicum and B. elkanii bacteroids.- Molecular control of alfalfa nodule NADH-glutamate synthase: A key for nitrogen assimilation.- Rearrangement of nif operons in cyanobacterial heterocysts.- Gene amplification in Rhizobium.- Molecular phylogeny of rhizobia and their relatives.- The prospects of increasing the input of biological nitrogen fixation in agriculture.- Endophytic diazotrophs in sugar cane, cereals and tuber plants.- Prospects and problems of biological nitrogen fixation in rice production: A critical assessment.- Diversity of photosynthetic Bradyrhizobium strains from stem nodules of Aeschynomene species.- Capacities of Azoarcus sp., a new genus of grass-associated diazotrophs.- Field testing of genetically engineered rhizobia.- Mechanism of nitrogenase catalysis.- Extension of nitrogen fixation to other crops.- Regeneration and transformation of legumes.- Competitiveness of Rhizobium strains for nodule occupancy.- Report from the Taxonomy Meeting, Subcommittee on Aqrobacterium and Rhizobium.- Author Index.