Enoyl-[acyl carrier protein] reductase (ENR) is an essential enzyme in type II fatty-acid synthesis that catalyzes the last step in each elongation cycle. Thus far FabI, FabL and FabK have been reported to carry out the reaction, with FabI being the most characterized. Some bacteria have more than one ENR, and Bacillus cereus has two (FabI and FabL) reported. Here, we have determined the crystal structures of the later in the apo form and in the ternary complex with NADP(+) and an indole naphthyridinone inhibitor. The two structures are almost identical, except for the three stretches that are disordered in the apo form. The apo form exists as a homo-dimer in both crystal and solution, while the ternary complex forms a homo-tetramer. The three stretches disordered in the apo structure are important in the cofactor and the inhibitor binding as well as in tetramer formation. (C) 2010 Elsevier Inc. All rights reserved.