The rpoS gene which encodes a stationary phase sigma factor has been identified and characterised from the rhizosphere-colonising plant growth-promoting Pseudomonas putida strain WCS358. The predicted protein sequence has extensive homologies with the RpoS proteins form other bacteria, in particular with the RpoS sigma factors of the fluorescent pseudomonads. A genomic transposon insertion in the rpoS gene was constructed, these mutants were analysed for their ability to produce siderophore (iron-transport agent) and the autoinducer quorum-sensing molecules called homoserine lactones (AHL). It was determined that RpoS was not involved in the regulation of siderophore and AHL production, synthesis of these molecules is important for gene expression at stationary phase. P. putida WCS358 produces at least three different AHL molecules.