Compared with other metals, molybdenum is rare in soil, fresh water, and marine environments (Hernandez et al., 2009). With few exceptions, however, molybdenum is required in most bacteria, archaea, and eukaryotes as an essential cofactor of enzymes involved in sulfur,

carbon, and nitrogen metabolism including nitrate reductase, xanthine dehydrogenase, DMSO reductase, and nitrogenase (Zhang & Gladyshev, 2008). Regulators belonging to the ModE family specifically sense and respond to molybdenum availability (Pau, 2004). Remarkably, ModE is found not only in bacteria but BMN 673 chemical structure also in archaea (Studholme & Pau, 2003; Zhang & Gladyshev, 2008). Cells take up molybdenum in its oxyanion form, molybdate (MoO42−). Often, modE genes are clustered with modABC genes coding for high-affinity molybdate (Mo) uptake systems, which consist of a periplasmic Mo-binding protein (ModA), a membrane-spanning Selleck MLN0128 transport protein (ModB), and the energizing cytoplasmic ATP-binding protein (ModC) (Self et al., 2001). Escherichia coli ModE is modular in structure as shown by X-ray crystallography (Hall et al., 1999). ModE consists of an N-terminal DNA-binding domain with a helix–turn–helix motif and a C-terminal Mo-binding domain. ModE forms dimers, which

bind to conserved palindromic sequences (Mo-boxes) within its target promoters (Anderson et al., 1997; Studholme & Pau, 2003). Upon Mo binding, conformation of ModE changes, and in turn, DNA affinity is increased (Anderson et al., 1997). Depending on the position of the Mo-box, ModE can either act as a repressor or as an activator of target gene transcription. For example, ModE represses the modABC operon (Grunden et al., 1996), thus preventing synthesis of the Mo-uptake system under Mo-replete conditions. On the other hand, ModE activates the moa genes involved in the synthesis of the molybdopterin cofactor (Moco) (McNicholas et al., 1997). Moco forms the active site

of all molybdoenzymes from bacteria, archaea, plants, and animals, except for molybdenum nitrogenases (Mo-nitrogenases), which contain an iron-molybdenum cofactor (FeMoco) (Rubio & Ludden, 2008). In contrast ASK1 to E. coli, the phototrophic alphaproteobacterium Rhodobacter capsulatus contains two modE-like genes: mopA and mopB (Wang et al., 1993; Wiethaus et al., 2006). MopA and MopB show 52% identity to each other, and each of these regulators is sufficient to repress several target genes including anfA, which codes for the activator of alternative (iron-only) nitrogenase (Fe-nitrogenase) genes. Both Fe-nitrogenase and Mo-nitrogenase catalyze the reduction of dinitrogen (N2) to ammonia (NH3) and thus enable R. capsulatus to grow with N2 as the sole source of nitrogen (Masepohl & Kranz, 2009). Mo-dependent repression of anfA prevents the synthesis of Fe-nitrogenase, which possesses lower specific activity than Mo-nitrogenase.