Conserved motifs encountered in the sequences of the molybdo-subunits ArrA, UnkA or Psr/PhsA. a. To the exeption of one subclass of the Unk sequences (see the text for more details), all three clades feature the TAT (twin-arginine translocation) signal sequence at the N-terminus. b. All three types of molybdo-subunits contain the motif for binding an iron-sulfur cluster (C-X2-C-X3-C-X27-C) near the N-terminus. c. Amino acid sequence alignment of the molybdenum-binding domain indicates a conserved cysteine residue which is likely to represent the amino acid that coordinates to the molybdenum.