Catalyzes the elimination of C-20 in precorrin-3A to form precorrin-3B. 2 H(+) + NADH + O2 + precorrin-3A = H2O + NAD(+) + precorrin-3B Cofactor biosynthesis; adenosylcobalamin biosynthesis; cob(II)yrinate a,c-diamide from precorrin-2 (aerobic route): step 2/10. Belongs to the nitrite and sulfite reductase 4Fe-4S domain family. Was originally thought to originate from Pseudomonas denitrificans, but similarity searches show that the sequence is much closer to Sinorhizobium. The entry's taxonomy has been changed.