NB-protein (BchN–BchB) of dark-operative protochlorophyllide reductase is the catalytic component containing oxygen-tolerant Fe–S clusters

Dark-operative protochlorophyllide (Pchlide) oxidoreductase is a nitrogenase-like enzyme consisting of the two components, L-protein (BchL-dimer) and NB-protein (BchN–BchB-heterotetramer). Here, we show that NB-protein is the catalytic component with Fe–S clusters. NB-protein purified from Rhodobacter capsulatus bound Pchlide that was readily converted to chlorophyllide a upon the addition of L-protein and Mg-ATP. The activity of NB-protein was resistant to the exposure to air. A Pchlide-free form of NB-protein purified from a bchH-lacking mutant showed an absorption spectrum suggesting the presence of Fe–S centers. Together with the Fe and sulfide contents, these findings suggested that NB-protein carries two oxygen-tolerant [4Fe–4S] clusters.