The Hennecke Lab - Symbiotic Nitrogen Fixation

The soil bacterium Bradyrhizobium japonicum has two principle life styles: free-living in soil or laboratory culture, and endosymbiotic within infected cells of soybean root nodules, where the bacterium performs nitrogenase-catalyzed fixation of molecular nitrogen for the benefit of the host. Concurrent with the conversion from free-living to symbiotic, the bacterium changes from an oxic to an extremely micro-oxic environment. Life in micro-oxia is possible because B. japonicum respires with the help of a high-affinity cytochrome oxidase that consumes the oxygen released slowly from oxy-leghemoglobin, an oxygen-binding protein of the host plant. The cytoplasm of the symbiotic bacterium is thus kept anoxic. This is an optimal niche for the function of the oxygen-sensitive nitrogenase. Research activity in the Hennecke group deals with many aspects of the physiology of the endosymbiont. Particular emphasis lies on processes in which the ambient oxygen concentration and the redox balance play critical roles in the regulation of gene expression and diverse symbiotic functions. The available genome sequence and tools for transcriptomics and proteomics facilitate systems biology approaches to understand complex intracellular networks in free-living and symbiotic B. japonicum.

Research projects

• Project 1: Regulatory networks »»
• Project 2: Molecular biology of root nodule bacteroids  »»
• Project 3: Analysis of symbiotically defective mutants  »»
• Project 4: Biogenesis of cytochrome oxidases  »»
• Special techniques and tools: B. japonicum gene chip  »»

Left: Root nodules resulting from the symbiotic interaction of Bradyrhizobium japonicum with its soybean host plant. Center: Section through a root nodule. The red color originates from the plant-derived protein leghemoglobin which reversibly binds oxygen. Right: Nitrogen-fixing B. japonicum bacteroids in an infected soybean root nodule cell.