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Cotranscription of genes encoding indoleacetic acid production in Pseudomonas syringae subsp. savastanoi

Indoleacetic acid (IAA) production by the plant pathogen Pseudomonas syringae subsp. savastanoi is essential for tumor formation on olive and oleander. The bacterium produces IAA from tryptophan in reactions catalyzed by tryptophan monooxygenase and indoleacetamide hydrolase. The genetic determinants are, respectively, iaaM and iaaH. In oleander isolates, the genes encoding the IAA biosynthetic enzymes are located on a plasmid; in olive isolates, the genes occur on the chromosome. The IAA genes from the oleander isolate strain EW2009 are located within a 4-kilobase (kb) segment of the 52-kb plasmid pIAA1. Escherichia coli strains harboring a recombinant plasmid, pCJP3, which contains this 4-kb fragment, excreted IAA into culture media, and crude cell extracts had both tryptophan monooxygenase and indoleacetamide hydrolase activity. In vitro coupled transcription-translation of pCJP3 demonstrated that this fragment coded for proteins of 62 and 47 kilodaltons which correspond to tryptophan monooxygenase and indoleacetamide hydrolase, respectively. Expression of these genes was dependent upon a vector promoter in pCJP3. However, in the absence of a vector promoter, E. coli containing recombinant plasmids with additional pIAA1 DNA in front of iaaM had high levels of tryptophan monooxygenase. Northern (RNA) hybridization experiments verified that iaaM and iaaH are cotranscribed as a portion of a ca. 4- to 5-kb transcript in vivo. Southern hybridization experiments with IAA plasmids from different oleander strains of P. syringae subsp. savastanoi revealed that all IAA plasmids contained a region of at least 10 kb of homology, with the IAA genes at one end. Repetitive DNA and a copy of IS51 were found at the end of this region of homology.