Cloning, expression, purification, crystallization and preliminary crystallographic analysis of NifH2 from Methanocaldococcus jannaschii

Nitrogenases are protein complexes that are only found in Azotobacter and are required for biological nitrogen fixation. They are made up of a nitrogenase, which is a NifD₂/NifK₂ heterotetramer, and a nitrogenase reductase, which is a homodimer of NifH. Many homologues of nitrogenase have been found in various non-nitrogen-fixing prokaryotes; in particular, they are found in all known methanogens. This indicates that these homologues may play a role in methane production. Here, the cloning of NifH2, a homologue of the NifH nitrogenase component, from Methanocaldococcus jannaschii (MjNifH2) and its expression in Escherichia coli with a polyhistidine tag, purification and crystallization are described. MjNifH2 crystals were obtained by the hanging-drop vapour-diffusion method and diffracted to a resolution limit of 2.85 Å. The crystals belonged to space group P2, with unit-cell parameters a = 64.01, b = 94.38, c = 98.08 Å, α = γ = 90, β = 98.85°.