Klebsiella pneumoniae is a member of the Enterobacteriaceae but unlike related organisms such as Escherichia coli it has the ability to fix nitrogen i.e. to convert atmospheric nitrogen gas to ammonium. This process, which is only found in bacteria, is of major global importance in the provision of fixed nitrogen for plant growth. Klebsiella is naturally found as a free-living soil bacterium and unlike other nitrogen fixers such as Rhizobium it does not participate in symbiotic interactions with leguminous plants. This ability to fix nitrogen in the free-living state has made it the organism of choice for studies of both the biochemistry and the genetics that underlie the nitrogen fixation process.
Nitrogen fixing (diazotrophic) organisms synthesise an enzyme called nitrogenase which is a complex oxygen-sensitive molybdoprotein containing a number of novel metal centres that are required for the binding of molecular nitrogen and its subsequent reduction to ammonia. Genetic studies in Klebsiella have identified a total of 20 genes (called nif genes) that are clustered in a 24kb region of the chromosome and required for the biosynthesis of nitrogenase and the regulation of its synthesis. The majority of these genes have been found to be common to all diazotrophs. Three of the genes, nifHDK, encode the three structural subunits of nitrogenase. The majority of the other genes, nifEN, nifUSVWX, nifM and nifBQ are required for various steps in the assembly and incorporation of the metal centres in nitrogenase. Two genes, nifF and nifJ encode proteins required for electron transfer to nitrogenase and two genes, nifLA, encode proteins that regulate nif gene expression.
Nitrogen fixation is in energy intensive process, requiring some 16
molecules of ATP for the reduction of one molecule of nitrogen to ammonia.
Given this requirement and the oxygen-sensitivity of the process Klebsiella
pneumoniae, which is a facultative anaerobe, only fixes nitrogen when
it is under anaerobic or microaerobic conditions and is nutritionally starved
of other sources of fixed nitrogen. The regulation of this process
occurs predominantly at the level of nif gene expression and the particularly
complex regulatory mechanisms involved are a major focus of our research.
These studies have revealed a number of novel biological mechanisms, including
a novel form of RNA polymerase with a unique sigma factor (sigma54 or sigmaN),
and a novel flavoprotein (NifL) that mediates nif gene regulation
in response to the cellular redox state. More details of current research
projects in this area can be obtained by reference to individual researchers
who work with Klebsiella.