Physiological control of amonabactin biosynthesis in Aeromonas hydrophila.

Amonabactin is a siderophore from Aeromonas hydrophila which is produced in two biologically active forms composed of the phenolate 2,3-dihydroxybenzoic acid (DHB), lysine, glycine, and either trytophan (amonabactin T) or phenylalanine (amonabactin P). Amonabactin biosynthetic mutants (generated by chemical mutagenesis) that either produced no amonabactin or overproduced the siderophore were isolated and identified on chrome azurol S siderophore detection agar. Amonabactin-negative mutants were of two categories. One type produced no phenolates and used exogenous DHB to synthesize amonabactin (both forms) while the other type excreted DHB but not amonabactin. This suggests an amonabactin biosynthetic pathway composed of two segments, one producing DHB and the other assembling amonabactin from DHB and the amino acids. Overproduction mutants used amonabactin poorly or not at all, indicating that they contained lesions in amonabactin utilization. Adding the analog D-tryptophan to wild-type A. hydrophila cultures reduced synthesis of both amonabactin T and amonabactin P and lengthened the lag phase in iron restricted medium. The tryptophan and phenylalanine forms of amonabactin may be synthesized by a single assembly pathway that contains a novel enzyme (sensitive to D-tryptophan) which inserts either tryptophan or phenylalanine into amonabactin.