arbutin and salicyl-alcohol

The salAB genes of Azospirillum irakense KBC1, which encode two aryl-beta-glucosidases, are required for growth on salicin. In the 4-kb region upstream of the salAB genes, two additional genes, salC and salR, were identified. SalC shows characteristics of the outer membrane receptors in the FepA/FhuA family. The salC AB genes are transcribed as a polycistronic mRNA. The salR gene encodes a protein homologous to the LacI/GalR family of transcriptional repressors. Expression of the sal operon, measured by means of a salC-gusA translational fusion in A. irkense KBC1, requires the presence of aryl-beta-glucosides such as arbutin and salicin. Expression is markedly enhanced when a simple carbon source, like glucose, cellobiose or malate, is added to the medium. In a salR mutant, expression of the salC-gusA fusion does not require an aryl-beta-glucoside inducer. Expression of a salR-gusA fusion is constitutive. The product of arbutin hydrolysis (hydroquinone) partly inhibits the expression of a salC-gusA fusion in arbutin- or salicin-containing minimal medium. This effect is independent of SalR. Salicylalcohol, the hydrolysis product of salicin, also partly inhibits salC expression in a SalR-independent fashion, but only in salicin-containing minimal medium.

Topics: Amino Acid Sequence; Arbutin; Azospirillum; Bacterial Proteins; Benzyl Alcohols; beta-Glucosidase; Carrier Proteins; Cloning, Molecular; Enzyme Induction; Gene Expression Regulation, Bacterial; Genes, Bacterial; Glucosides; Hydrolysis; Hydroquinones; Molecular Sequence Data; Operon; Receptors, Cell Surface; Repressor Proteins; Sequence Analysis, DNA; Sequence Homology, Amino Acid; Transcription, Genetic

The syrB gene is required for syringomycin production by Pseudomonas syringae pv. syringae and full virulence during plant pathogenesis. Strain B3AR132 containing a syrB::lacZ fusion was used to detect transcriptional activation of the syrB gene in syringomycin minimal medium by plant metabolites with signal activity. Among 34 plant phenolic compounds tested, arbutin, phenyl-beta-D-glucopyranoside, and salicin were shown to be strong inducers of syrB, giving rise to approximately 1,200 U of beta-galactosidase activity at 100 microM; esculin and helicin were moderate inducers, with about 250 to 400 U of beta-galactosidase activity at 100 microM. Acetosyringone and flavonoids that serve as signal molecules in Agrobacterium and Rhizobium species, respectively, did not induce the syrB::lacZ fusion. All syrB inducers were phenolic glucosides and none of the aglucone derivatives were active, suggesting that the beta-glycosidic linkage was necessary for signal activity. Phenyl-beta-D-galactopyranoside containing galactose substituted for glucose in the beta-glycosidic linkage also lacked inducer activity. Phenolic signal activity was enhanced two- to fivefold by specific sugars common to plant tissues, including D-fructose, D-mannose, and sucrose. The effect of sugars on syrB induction was most noticeable at low concentrations of phenolic glucoside (i.e., 1 to 10 microM), indicating that sugars such as D-fructose increase the sensitivity of P. syringae pv. syringae to the phenolic plant signal. Besides induction of syrB, syringomycin biosynthesis by parental strain B3A-R was induced to yield over 250 U of toxin by the additions of arbutin and D-fructose to syringomycin minimal medium. These data indicate that syringomycin production by most strains of P. syringae pv. syringae is modulated by the perception of two classes of plant signal molecules and transduced to the transcriptional apparatus of syringomycin (syr) genes such as syrB.

Topics: Arbutin; Bacterial Proteins; Benzyl Alcohols; Fructose; Gene Expression Regulation, Bacterial; Glucosides; Hydroquinones; Phenols; Plant Physiological Phenomena; Pseudomonas; Signal Transduction; Transcription, Genetic