Component of the dark-operative protochlorophyllide reductase (DPOR) that uses Mg-ATP and reduced ferredoxin to reduce ring D of protochlorophyllide (Pchlide) to form chlorophyllide a (Chlide). This reaction is light-independent. The NB-protein (BchN-BchB) is the catalytic component of the complex (By similarity). 2 ADP + chlorophyllide a + oxidized 2[4Fe-4S]-[ferredoxin] + 2 phosphate = 2 ATP + 2 H2O + protochlorophyllide a + reduced 2[4Fe-4S]-[ferredoxin] Binds 1 [4Fe-4S] cluster per heterodimer. The cluster is bound at the heterodimer interface by residues from both subunits. Porphyrin-containing compound metabolism; bacteriochlorophyll biosynthesis (light-independent). Protochlorophyllide reductase is composed of three subunits; BchL, BchN and BchB. Forms a heterotetramer of two BchB and two BchN subunits (By similarity). Belongs to the ChlB/BchB/BchZ family.