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 L component serves as a unique electron donor to the NB-component of the complex, and binds Mg-ATP. 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 dimer. Porphyrin-containing compound metabolism; chlorophyll biosynthesis (light-independent). Homodimer. Protochlorophyllide reductase is composed of three subunits; ChlL, ChlN and ChlB. Belongs to the NifH/BchL/ChlL family.