Indoleacetic acid operon of Pseudomonas syringae subsp. savastanoi: transcription analysis and promoter identification

Expression of the indoleacetic acid (iaa) operon, which contributes to the virulence of the phytopathogenic bacterium Pseudomonas syringae subsp. savastanoi, was monitored by using broad-host-range lacZ reporter gene plasmids. A combination of translational (gene) fusions and transcriptional (operon) fusions of P. syringae subsp. savastanoi sequences to lacZ allowed localization of the iaa operon promoter. RNA recovered from P. syringae subsp. savastanoi strains was mapped with iaa operon-specific probes to precisely locate the transcription initiation site. When transcripts from an iaaM::lacZ fusion in Escherichia coli were analyzed, an identical transcription initiation site was observed. The DNA sequence of the iaa operon promoter closely resembled the consensus E. coli promoter sequence. We detected an active, constitutive level of indoleacetic acid biosynthetic gene expression during bacterial growth under a variety of conditions in the absence of host plant influence.