Chaperone required for proper molybdenum cofactor insertion and final assembly of the membrane-bound respiratory nitrate reductase 1. Required for the insertion of the molybdenum into the apo-NarG subunit, maybe by keeping NarG in an appropriate competent-open conformation for the molybdenum cofactor insertion to occur. NarJ maintains the apoNarGH complex in a soluble state. Upon insertion of the molybdenum cofactor, NarJ seems to dissociate from the activated soluble NarGH complex, before its association with the NarI subunit on the membrane. Binds specifically to the NarG subunit of the apoenzyme complex at two distinct sites, one interfering with membrane anchoring and another being involved in molybdenum insertion. Belongs to the NarJ/NarW family.