The NosX and NirX proteins of Paracoccus denitrificans are functional homologues: Their role in maturation of nitrous oxide reductase

The nos (nitrous oxide reductase) operon of Paracoccus denitrificans contai ns a nosX gene homologous to those found in the nos operons of other denitr ifiers. NosX is also homologous to NirX, which is so far unique to P. denit rificans. Single mutations of these genes did not result in any apparent ph enotype, but a double nosX nirX mutant was unable to reduce nitrous oxide. Promoter-lacZ assays and immunoblotting against nitrous oxide reductase sho wed that the defect was not due to failure of expression of nosZ, the struc tural gene for nitrous oxide reductase. Electron paramagnetic resonance spe ctroscopy showed that nitrous oxide reductase in cells of the double mutant lacked the Cu, center. A twin-arginine motif in both NosX and NirX suggest s that the NosX proteins are exported to the periplasm via the TAT transloc on.