The NosX and NirX proteins of Paracoccus denitrificans are functional homologues: their role in the maturation of nitrous reductase.

The nos (nitrous oxide reductase) operon of Paracoccus denitrificans contains a nosX gene homologous to those found in the nos operons of other denitrifiers. NosX is also homologous to NirX, which is so far unique to P. denitrificans. Single mutations of these genes did not result in any apparent phenotype, but a double nosX nirX mutant was unable to reduce nitrous oxide. Promoter-lacZ assays and immunoblotting against nitrous oxide reductase showed that the defect was not due to failure of expression of nosZ, the structural gene for nitrous oxide reductase. Electron paramagnetlc resonance spectroscopy showed that nitrous oxide reductase in cells of the double mutant lacked the Cu(A), center. A twin-arginine motif in both NosX and NirX suggests that the NosX proteins are exported to the periplasm via the TAT translocon.