A second narGYI cluster learn more (Figure 5b; Gmet_1020 to Gmet_1022) is missing a noncatalytic subunit (narJ), and its expression has not been detected (B. Postier, personal communication). The first gene of both operons encodes a unique diheme c-type cytochrome (Gmet_0328 and Gmet_1019), suggesting that the nitrate reductase may be connected to other electron transfer components besides the menaquinol pool, perhaps operating in reverse as a nitrite oxidase. The product of the ppcF gene (Gmet_0335) in the intact nar operon, which is related to a periplasmic triheme c-type cytochrome involved in Fe(III) reduction in G. sulfurreducens , may permit electron transfer to
the nitrate reductase from extracellular electron donors such as humic substances  or graphite electrodes . The final two genes of the intact selleck nar Eltanexor research buy operon (Gmet_0336-Gmet_0337), encode the MoeA and MoaA enzymes implicated in biosynthesis of bis-(molybdopterin guanine dinucleotide)-molybdenum, an essential cofactor of the nitrate reductase. Figure 5 The respiratory nitrate reductase operons. (a) The major (expressed) operon also encodes the nitrate and
nitrite transporters (narK-1, narK-2), two c-type cytochromes including ppcF, and two genes of molybdenum cofactor biosynthesis (moeA-2, moaA-2). (b) The minor operon (expression not detected) also encodes the Rieske iron-sulfur component of nitrite reductase (nirD) and a c-type cytochrome, but lacks a narJ gene. Phylogenetic analysis indicates that the moeA and moaA gene families have repeatedly expanded in various Geobacteraceae (data not shown). G. sulfurreducens has a single copy of each, but G. metallireducens has three closely related isoenzymes, of which moeA-1 (Gmet_1038 = GSU2703,
40% identical to the E. coli protein ) and moaA-1 (Gmet_0301 = GSU3146, 36% identical to the E. coli protein ) occupy a conserved location among other genes of molybdopterin biosynthesis (Table 1, Figure 6). A possible reason for the expansion in G. metallireducens and other Geobacteraceae is a need to upregulate molybdopterin biosynthesis for specific processes: moeA-2 and moaA-2 (Gmet_0336-Gmet_0337, 38% and 33% identity Ergoloid to the E. coli proteins) may support nitrate reduction; moaA-3 (Gmet_2095, 35% identity to E. coli) may function with nearby gene clusters for catabolism of benzoate  and p-cresol ; and moeA-3 (Gmet_1804, 37% identity to E. coli) may aid growth on benzoate, during which it is upregulated . G. metallireducens differs from G. sulfurreducens in other aspects of molybdenum assimilation as well (Table 1): notably, G. sulfurreducens possesses a homolog of the moaE gene (GSU2699) encoding the large subunit of molybdopterin synthase, but lacks homologs of the small subunit gene moaD and the molybdopterin synthase sulfurylase gene moeB, whereas G.