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 (ChlN-ChlB) is the catalytic component of the complex. 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; chlorophyll biosynthesis (light-independent). Protochlorophyllide reductase is composed of three subunits; ChlL, ChlN and ChlB. Forms a heterotetramer of two ChlB and two ChlN subunits (Probable). Cells lacking this gere are unable to synthesize chlorophyll, accumulating protochlorophyllide in darkness while synthesizing chlorophyll normally in the light. Belongs to the ChlB/BchB/BchZ family.