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The phytopathogen Pseudomonas syringae subsp. savastanoi incites the production of galls on olive and oleander plants. Gall formation is dependent on bacterial production of the phytohormone indoleacetic acid (IAA). The genetic determinants for IAA synthesis are located on a plasmid (pIAA) and are organized in an operon in oleander strains of the bacterium. P. syringae subsp. savastanoi further converts IAA to an amino acid conjugate, 3-inidole-acetyl-r-L-lysine (IAA-lysine). The gene for IAA-lysine synthetase (iaaL) was found on the IAA plasmid by screening pIAA deletion mutants for the ability to convert IAA to IAA-lysine. The iaaL locus was then cloned in the vector pUC8 from a bank of P. syringae subsp. savastanoi EW2009 plasmid DNA to construct recombinant plasmid pLG87. The specific activity of IAA-lysine synthetase in Escherichia coli transformed with pLG87 was 47 times higher than that of the enzyme extract from P. syringae subsp. savastanoi. The direction of transcription of the iaaL gene was determined to be opposite to that of the IAA operon. The location of the iaaL gene on pIAAl was mapped by TnS insertion mutagenesis to a 2.5-kilobasepair fragment 2 kilobase pairs from the IAA operon.Pseudomonas syringae subsp. savastanoi (Smith) Stevens incites a disease of oleander (Nerium oleander L.) and olive (Olea europa L.) that is characterized by tumorous outgrowths on the stems and leaves of infected plants (21,26,27). This development of galls is dependent on bacterial production of the phytohormone indoleacetic acid (IAA) (6, 23). The two enzymes involved in the conversion of tryptophan to IAA are tryptophan monooxygenase, which converts tryptophan to indoleacetamide, and indoleacetamide hydrolase, which catalyzes the conversion of indoleacetamide to IAA (16,17). IAA can be further metabolized to an amino acid conjugate, 3-indole-acetyl-e-L-lysine (IAA-lysine) (12, 13).The genes for tryptophan monooxygenase, iaaM, and indoleacetamide hydrolase, iaaH, are organized in an operon in P. syringae subsp. savastanoi (7,8,28); the iaaM locus is the first gene transcribed. In strains isolated from oleander galls, the IAA operon is located on a plasmid (pIAA) (6, 9). Oleander gall isolates selected for resistance to the tryptophan analog a-methyl tryptophan are often incapable of synthesizing IAA and cannot incite the production of galls when inoculated onto oleander plants (23). These avirulent IAA-mutants fall into two classes when surveyed for their plasmid composition: those that lack the IAA operon, either by the loss of the IAA plasmid or by deletion of the IAA genes, and those that contain insertions in the iaaM gene (6,8,9).Certain isolates of P. syringae subsp. savastanoi convert IAA to 13