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Members of Methylobacteriumpigmented facultative methylotrophic bacteria, are frequentlyasociated with terrestrial and aquatic plants, tending to formaggregates on the phyllosphere. We report here that the productionof autoinducer molecules involved in the cel-to-cel signalingproces, which is known as quorum sensing, is common amongMethylobacterium species. Several strains of Methylobacteriumwere tested for their ability to produce N-acyl-homoserineMost strains of Methylobacterium tested could elicit apositive response in Agrobacterium tumefaciens harboringlacZ fused to a gene that is regulated by autoinduction. Thesynthesis of these compounds was cell-density dependent, andthe maximal activity was reached during the late exponentialto stationary phases. The bacterial extracts were separated bythin-layer chromatography and bioassayed with A. tumefaciensNT1 (traR, tra:lacZ749various patterns of the signal molecules, which are straindependent. At least two signal molecules could be detected inmost of the strains tested, and comparison of their relativemobilities suggested that they are homologs of N-octanoyl-DL-homoserine lactone (C8-HSL) and N-decanoyl-DL-homoserinelactone (C10-HSL).