(a) Cladograms of proteins containing the IPR003731 domain. Coloured clades contain members of the NifB (IPR005980, blue), NifX (IPR013480, turquoise) and NafY (IPR031763, purple) InterPro protein families. The proposed IssA clade (red) is based on predicted protein isoelectric point (pI) and occurrence of proteins with a glycine rich region at the C-terminus. P. furiosus IssA and the homologue from Methanothermobacter thermautotrophicus (MTH1175) are shown by red and yellow arrowheads respectively. (b) Modelled structure of the IssA IPR003731 domain based on the NMR structure of MTH1175 (red). The C-terminal region is shown unstructured, as predicted for the apo-form.