Thiamine-Auxotrophic Mutants of Pseudomonas fluorescens CHA0 Are Defective in Cell-Cell Signaling and Biocontrol Factor Expression

• Published in: Applied and Environmental Microbiology 2006-04, Vol.72 (4), p.2606-2613
• Main Authors: Dubuis, Christophe, Rolli, Joëlle, Lutz, Matthias, Défago, Geneviève, Haas, Dieter
• Format: Article
• Language: English
• Subjects: Amino Acid Sequence; auxotrophs; Bacteria; Bacterial Proteins - genetics; Bacterial Proteins - metabolism; Base Sequence; Biological and medical sciences; biological control; biological control agents; Brassicaceae - microbiology; cell communication; disease control; DNA Transposable Elements; Enzymes; Fundamental and applied biological sciences. Psychology; fungal antagonists; fungal diseases of plants; Gene expression; Gene Expression Regulation, Bacterial; genes; Genetics and Molecular Biology; Lepidium sativum; Microbiology; Molecular Sequence Data; Mutagenesis, Insertional; mutants; Mutation; Pest Control, Biological; phenotype; Plant Diseases - microbiology; Pseudomonas fluorescens; Pseudomonas fluorescens - genetics; Pseudomonas fluorescens - growth & development; Pseudomonas fluorescens - metabolism; Pythium - growth & development; Pythium ultimum; Recombinant Fusion Proteins - genetics; Recombinant Fusion Proteins - metabolism; regulatory RNA; Repressor Proteins - genetics; Repressor Proteins - metabolism; RNA; RNA, Bacterial - metabolism; rsmZ gene; Signal Transduction; thiamin; Thiamine - metabolism
• ISSN: 0099-2240; 1098-5336
• DOI: 10.1128/AEM.72.4.2606-2613.2006

In the biocontrol strain Pseudomonas fluorescens CHA0, the Gac/Rsm signal transduction pathway positively controls the synthesis of antifungal secondary metabolites and exoenzymes. In this way, the GacS/GacA two-component system determines the expression of three small regulatory RNAs (RsmX, RsmY, and RsmZ) in a process activated by the strain's own signal molecules, which are not related to N-acyl-homoserine lactones. Transposon Tn5 was used to isolate P. fluorescens CHA0 insertion mutants that expressed an rsmZ-gfp fusion at reduced levels. Five of these mutants were gacS negative, and in them the gacS mutation could be complemented for exoproduct and signal synthesis by the gacS wild-type allele. Furthermore, two thiamine-auxotrophic (thiC) mutants that exhibited decreased signal synthesis in the presence of 5 x 10⁻⁸ M thiamine were found. Under these conditions, a thiC mutant grew normally but showed reduced expression of the three small RNAs, the exoprotease AprA, and the antibiotic 2,4-diacetylphloroglucinol. In a gnotobiotic system, a thiC mutant was impaired for biological control of Pythium ultimum on cress. Addition of excess exogenous thiamine restored all deficiencies of the mutant. Thus, thiamine appears to be an important factor in the expression of biological control by P. fluorescens.