Hierarchical autoinduction in Ralstonia solanacearum: control of acyl-homoserine lactone production by a novel autoregulatory system responsive to 3-hydroxypalmitic acid methyl ester.
Bacteria employ autoinduction systems to sense the onset of appropriate cell density for expression of developmental genes. In many gram-negative bacteria, autoinduction involves the production of and response to diffusible acylated-homoserine lactones (acyl-HSLs) and is mediated by members of the LuxR and LuxI families. Ralstonia (Pseudomonas) solanacearum, a phytopathogenic bacterium that appears to autoregulate its virulence genes, produces compounds that promote expression of several heterologous acyl-HSL-responsive reporter gene constructs. High-pressure liquid chromatography of highly concentrated ethyl acetate extracts revealed that culture supernatants of strain AW1 contained two compounds with retention times similar to N-hexanoyl- and N-octanoyl-HSL. To investigate the role of these acyl-HSLs in R. solanacearum virulence gene expression, transposon mutants that were deficient for inducing an acyl-HSL-responsive reporter in Agrobacterium tumefaciens were generated. Three loci involved in normal acyl-HSL production were identified, one of which was shown to contain the divergently transcribed solR and solI genes, the luxR and luxI homologs, respectively. A 4.1-kb fragment containing solR and solI enabled all of the mutants (regardless of the locus inactivated) and a naturally acyl-HSL-defective strain of R. solanacearum to produce acyl-HSLs. Inactivation of solI abolished production of all detectable acyl-HSLs but affected neither the expression of virulence genes in culture nor the ability to wilt tomato plants. AW1 has a functional autoinduction system, because (i) expression of solI required SolR and acyl-HSL and (ii) expression of a gene linked to solR and solI, designated aidA, was acyl-HSL dependent. Because AidA has no homologs in the protein databases, its discovery provided no clues as to the role of acyl-HSLs in R. solanacearum gene regulation. However, expression of solR and solI required the global LysR-type virulence regulator PhcA, and both solR and solI exhibited a cell density-associated pattern of expression similar to other PhcA-regulated genes. The acyl-HSL-dependent autoinduction system in R. solanacearum is part of a more complex autoregulatory hierarchy, since the transcriptional activity of PhcA is itself controlled by a novel autoregulatory system that responds to 3-hydroxypalmitic acid methyl ester.
Flavier
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AB
Department of Plant Pathology, University of Georgia, Athens 30602-7274, USA.
Ganova-Raeva
L M
LM
Schell
M A
MA
Denny
T P
TP
D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000032:analysis / Q000302:isolation & purification / Q000378:metabolism; D016960:Agrobacterium tumefaciens / Q000235:genetics; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics* / Q000378:metabolism; D001483:Base Sequence; D002851:Chromatography, High Pressure Liquid; D003001:Cloning, Molecular; D004251:DNA Transposable Elements; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D017930:Genes, Reporter; D016946:Gram-Negative Aerobic Rods and Cocci / Q000235:genetics* / Q000378:metabolism* / Q000472:pathogenicity; D008969:Molecular Sequence Data; D010169:Palmitic Acids / Q000378:metabolism*; D010957:Plasmids; D011995:Recombination, Genetic; D012097:Repressor Proteins; D016415:Sequence Alignment; D017422:Sequence Analysis, DNA; D017386:Sequence Homology, Amino Acid; D015534:Trans-Activators; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic; D014774:Virulence / Q000235:genetics
D001426:Bacterial Proteins; D004251:DNA Transposable Elements; D004268:DNA-Binding Proteins; C085427:LuxI protein, Bacteria; D010169:Palmitic Acids; C082933:PhcA protein, Ralstonia solanacearum; D012097:Repressor Proteins; C109192:SolI protein, Ralstonia solanacearum; C109193:SolR protein, Ralstonia solanacearum; D015534:Trans-Activators; D014157:Transcription Factors; C064778:LuxR autoinducer binding proteins; C088386:homoserine lactone; D015107:4-Butyrolactone