Department of Biochemistry, McGill University, Montreal, Quebec, Canada.
The diversion of fatty acids from fatty acid biosynthesis into the luminescent system is catalyzed by a lux-specific acyltransferase that catalyzes the cleavage of fatty acyl-acyl carrier protein (ACP). Analysis of the substrate specificities for fatty acyl-ACPs of the transferases from divergent luminescent bacteria, Photobacterium phosphoreum and Vibrio harveyi, has demonstrated that myristoyl-ACP is cleaved at the highest rate. Inhibition by phenylmethanesulfonyl fluoride as well as resistance of the acylated enzyme intermediate to cleavage by hydroxylamine showed that the transferase is a serine esterase. Moreover, activity was dependent on a basic residue with a pKa of 6.3 implicating a histidine residue as part of a charge relay system found in serine esterases. The nucleotide sequence of the P. phosphoreum luxD gene coding for the transferase was determined resulting in the identification of the active site motif for serine esterases, G-X-S-X-G. Replacement of the serine residue at the center of this motif by threonine, alanine, or glycine blocked the transferase acyl-ACP cleavage activity, its ability to be acylated, and complementation of a transferase defective mutant on transconjugation with the luxD gene. The sequence and location of the serine as well as a histidine residue in the lux-specific transferases were found to be similar to those involved in the charge relay system in vertebrate thioesterases. Combined with the similar kinetic properties, these results support a common metabolic role for both enzymes in the diversion of fatty acids from the fatty acid biosynthetic pathway.