Rhodobacter sphaeroides contains two enoyl-acyl carrier protein (ACP) reductases, FabI(1) and FabI(2). However, FabI(1) displays most of the cellular enzyme activity. The spontaneous diazaborine-resistant mutation was mapped as substitution of glutamine for proline 155 (P155Q) of FabI(1). The mutation of FabI(1)[P155Q] increased the specificity constants (k(cat)/K-m) for crotonyl-ACP and NADH by more than 2-fold, while the site-directed mutation G95S (FabI(1)[G95S]), corresponding to the well-known G93 mutation of Escherichia coli FabI, rather decreased the values. Inhibition kinetics of the enzymes revealed that triclosan binds to the enzyme in the presence of NAD+, while the diazaborine appears to interact with NADH and NAD+ in the enzyme active site. The apparent inhibition constant K-i' of triclosan for FabI(1)[P155Q] and FabI(1)[G95S] at saturating NAD(+) were approximately 80- and 3-fold higher than that for the wild-type enzyme, respectively, implying that the inhibition was remarkably impaired by the P155Q mutation. The similar levels of K; of diazaborine for the mutant enzymes were also observed with respect to NAD+. Thus, the novel mutation P155Q appears to disturb the binding of inhibitors to the enzyme without affecting the catalytic efficiency. (C) 2002 Elsevier Science (USA). All rights reserved.